How can normal blood pressure suddenly rise and stay on the high side

130/80 is considered normal for those of us over 50 because our blood vessels are a little less flexible. Your diastolic is within range and I'm a little surprised that you were prescribed BP medicine. Thank goodness for side effects (sometimes).
The systolic the top number varies from minute to minute between 5 and10 mmhg. Coffee or tea (caffeine) can raise the systolic number 14 mmhg.
If sitting completely still for 30 minutes is difficult, you must be normal. Forcing yourself to do so can raise your blood pressure.
158 is too high though the risk of stroke is said to be at 180. More importantly the 158 seems to be an anomaly.
And they want three reading because often the home devices aren't altogether accurate.
I don't think you should be worried at all. But you could get a cardiac consult. I decided to because of my age-- The cardiologist I saw doesn't consider prescribing medication for a systolic under 140 in our demographic.
From 1991 https://pmc.ncbi.nlm.nih.gov/articles/PMC9697712/#:~:text=A%20study%20found%20that%20there%20is%20a,times%20higher%20hazard%20ratio%20for%20previous%20osteoporosis

I bet your doctor's nurse doesn't give you time to relax after you walk into the exam room. I know mine doesn't.

@gently Thanks. I didn't agree with my doctor either on the 130/80. I did see a cardiologist, and he wasn't worried. I asked him to reach out to my GP and explain things to her.
I believe the 158 was a fluke. I waited about 5 minutes to continue. I think just have to take my BP everyday is stressful.

There was a time when 120/80 was a perfectly acceptable B/P. Everything seems to have been downgraded lately with a desire to make people more "normal" even as we all get older and our poor circulatory systems are getting more "challenged." My husband got a diagnosis of "prediabetes" even though his A1C was in the normal range, but at the top end! I got a glucometer for home use and he is never over 100 for fasting blood glucose. Another diagnosis, another something for the PCP to fuss over! Anyway, in my experience, and I am a nurse, you don't sound so hypertensive as hypersensitive. Take your B/P at home but don't get too concerned about all the instructions. Go to YouTube and watch a video called 10 things that are done incorrectly when taking a B/P and learn that most of the time, the people in the doctor's office do it wrong! The cuff shouldn't be too tight, the arm should be a heart level, the pressure should be taken on the Left arm, the pressure cuff should not be placed over clothing, the B/P should not be taken when you first go into the exam room (I have white coat syndrome so I already know that my b/p will be off the wall), the first reading you take at home should be discarded, wait a couple minutes and then take it again. Take it the same time every day since it fluctuates with what you are doing or what is going on for you. In other words, you are allowing them to make you nuts by worrying about the numbers which in turn makes the numbers higher. And, I don't know if this is an issue, but if you lose 5 to 10 pounds if you are over average weight, and /or exercise regularly, your b/p will likely improve significantly. Relax and not only will your readings will be better, but you won't be stressing your system as much! Good luck and keep smiling! I am 79 and I know of what I say! 🙂

@penstruck The general rule of thumb is to take three separate measurements a few minutes apart. Almost universally each successive result will be a lower pair of figures showing BP. I would take the lowest of the three as the real value, but averaging them might be a better indicator of your 'daytime' BP. Notice that doctors and their nurses never do that? You get one cuff measurement and then comes the raised eyebrow.
We have three cuffs at home, one of them well worn and it seems to show the lowest scores routinely. The newest one, rechargeable and only a reader/display on a cuff, no tubes or wires, reads a bit higher than the other two. So, it could be that the newer cuffs will tend to show higher scores. Mebbe? But you have the luxury of being able to take three readings several minutes apart. The idea is to sit still, unlax, think about bed, a nap, a walk...nothing that might remind you of obligations or troubles...and then take three measurements. The lowest is probably what you have normally when not flustered, embarrassed, angry, walking, etc.

Thank you. I did not know to wait a few moments between the readings. That will help with the stress.
You made me laugh with "then comes the raised eyebrow".
I think my cuff is too big, or I am just not paying attention. I have difficulty getting the Velcro to stick; the cuff slips down and sideways as I try to adjust it. I try to get it to a good fit and leave it, but the cuff still comes undone.
I think the three readings, a few minutes apart, will help with the accuracy.

@sjm46 Thanks so much. This really helps. Yes, I can feel the stress rising every time I get ready to sit still and take the pressure. I will look for the YouTube video.
Since I have this new doctor, she has sent me to a cancer doctor over my platelets. He cided me, and I told him to cide her. I have constant blood work.
I have actually lost 7 pounds since this all started. I exercise (on a treadmill) almost every day, and I watch my sodium intake, which is difficult and tasteless.
I am concerned she thinks older adults are always on the verge of death.

Yes, you are so right! Every visit a new diagnosis! Had a bone scan that showed mild osteopenia! This is not even a real condition! 70% of people over 70 have some osteopenia! So young PCP messages me immediately with wanting to put me on a bone building drug! First of all, those drugs don't necessarily work, have lots of bad side effects and are not the first way to treat any form of bone loss! Like you, I exercise, eat right and take some calcium supplements. I just increased my calcium intake after some research online and I told her No to any drugs (probably promoted by Big Pharma!) So just try to stay calm (OHMM) that's what my husband and I do when we sit down each morning to compare B/P readings! LOL. Good luck and keep smiling! 😁

I just read an article that it should be placed on left arm always and arm should be at heart level and fit comfortably. You don't have to sit still for 30 minutes .... just not running around prior to taking. I take mine consistently at kitchen table with arms resting on table and it logs for me in my "new Kardia" ekg tiny monitor. I take both 3x a day. I use the "Omron" BP and it is pretty accurate as I've tested it at dr. office. AND recently nurse "requested" to use my right arm and I declined and said no please use my left. Good luck.

@penstruck The cuff should be modestly snug, but not 'snug'. It is supposed to take up any gaps by inflating, over-inflating, and then gradually releasing the pressure while the sensor is more firmly pressed against the radial artery, just above your inner elbow.
When I calibrate my Samsung Galaxy watch for its BP measurement app (also does ECG, blood O2 sat, and the newest version does skin free radical assessment to provide guidance on better eating habits...yes, it's true!), the instructions say to use the calibrating cuff on the opposite arm....the OPPOSITE arm...of the one where the watch is located. If you think about it a bit, in order to get certified by the (Canadian in my case) government to have the app activated and available to customers, the data must show the certifying body that the measurements by the watch are highly veridical. That is to say, they closely match the regular cuffs when they are calibrated with one of the widely available cuffs sold here and there. If the cuff is to be worn on the opposite arm, then the two arms must be highly correlated in terms of their resulting measured BP. I'm not saying there is absolutely no difference; down a flexible pipe of endothelial and lumen tissue that comprises blood vessels, you'll get lower and lower readings the further you get away from the heart. But the cuff on one arm and the watch on the other are pretty close, and at about the same height as well. In fact, the cuff is about six inches higher if both arms are properly resting on a table in front of you.