Well denimart, sorry to say, I don't really have good news for you.
Everything points to "Primary Exertional Headaches" and/or "Thunderclap Headache" which I previously described to you. PEH occurs exactly as stated with "exertion". Such as tensing or straining the abdominal muscles (being constipated and trying to poop), heavy lifting, coughing, sneezing and, as you unfortunately found out, an orgasm. Usually lasting seconds to minutes but with repeated exertion may build and become longer in duration. When first evaluating a patient, a CT scan or brain MRI is done. You had the CT scan already, showing nothing...personally I think a brain MRI would have been better. If all looks well in your brain, then it is called "primary" as oppose to "secondary" which would be for example...a person experiencing a massive headache in response to a stroke. I believe, and I am no neurologist!, you are describing a "thunderclap headache" especially since you said you develop high blood pressure along with terrible head pain.
How high is high? There is such a condition as "hypertensive crisis" which warrants a visit to the ER. This is a severe, sudden blood pressure in the range of 180/120 or more brought on by many possible causes which have to be ruled out. But with your CT scan, conditions such as: a bleed in the brain, rupture of a blood vessel in the brain, or a tear in the lining of an artery, blood clot in the brain, stroke, brain infections, etc. - have all been ruled out. Which leaves standing - a sudden, severe jump in your blood pressure. And you have discovered a hard strain of your abdominal muscles - as in trying to expel constipated hard stools causes that severe, sudden increase in B/P and what follows is that terrible, exploding, frightening headache. A "Thunderclap".
And as well - the "pushing" to have an orgasm will increase your B/P so severely that it causes a terrible, massive thunderclap headache. Do you have a history of migraines (there are several types)? Did you ever suffer trauma to your head as in a fall, car accident, etc.? Did you every suffer a concussion? Symptoms of which can show up months after the initial head trauma. It took 11 months for my symptoms (a thunderclap headache) to develop after my concussion. And as I reported, I was dehydrated (never good) and exercising like a mad woman PLUS I had a history of head trauma in a fall PLUS I suffer from migraines - as a result of that concussion. Whew!
You say you have mild headaches for days post severe headache (thunderclap headache). Believe it or not, I had a migraine for 9 months after my thunderclap headache...tried a multitude of medications, nothing helped but time. In a thunderclap headache the blood vessels in the brain suddenly, severely EXPAND, your B/P goes sky high and BOOM! My brain MRI (at the time 6 years ago) was normal. Now at age 70 it is not. My personal opinion is that I kick started this condition after my head trauma and because of that - when I was dehydrated (the brain is mostly water), fatigued and hitting a punching bag like Muhammad Ali - I had my Thunderclap!!
What to do? Get a "Headache Specialist" - all such specialists are neurologists but not ALL neurologists are headache specialists. Watch the level and intensity of any exercise you do. Slow and steady wins the race. Keep full hydrated at all times. Take a stool softener or increase fiber (docusate/Metamucil) to AVOID those hard stools...they are your enemy. Watch your level of fatigue. Continue to monitor your blood pressure recordings. Naturally avoid stress as much as possible, ever think of trying meditation? Yoga? Now what can I say about sex?
I had to bite the bullet and simply give up (not sex) but the effort to push through an orgasm. I continue to have our "version" of sex but without the orgasm for me. Fair? Heck, no! But I never want to experience that thunderclap headache ever ever again. And I haven't. And the way I figure it - after my divorce in my 50s I got involved with a sex obsessed man (who was into kinky) and I had sex constantly! It was fun while it lasted...so I feel I had my fill of wild and crazy sex. Now with my beloved, wonderful 2nd husband - we go nice and easy and improvise. And it works! And your question as to the WHY does this happen to me...review my questions, see if you are there for a possible, probable causes. If not, I was told once by a neurologist that so much is still unknown about the brain. I have simply accepted this is my life now. I believe I know my "why"...I would (again) strongly suggest a new headache specialist/neurologist for you. And see if you can get a brain MRI...it is the gold standard in testing the brain. All the best to you! Keep me updated and be well! 🙂