Repeat readings above 200 are a good reason to call the doctor, that's the guideline I was given by my endo.

Stress and illness can cause high readings. Strangely I've found a long drive in a car can do the same thing, even if I don't really notice any stress.

When I get some high readings I crack down on my diet, reduce the carb count to something under twenty per meal, or even fast plus or minus a celery stick or something. Stop cheating for a day or two. And that has worked for me. Stay hydrated with water, or some green tea.

If you are tired and thirsty, yes, those are the symptoms of high BG. While in that state do not exercise vigorously, that can send you into ketoacidosis which can do real damage.

OTOH this is not a reason to panic, days, weeks, even months at those levels are not uncommon before people are diagnosed. Somewhere over 250 or even 300, you might get to panic levels, but even those are not ambulance time in most cases, urgent care should be sufficient.

What diabetes meds are you on? And have you changed any other meds recently?