Hi, this could be due to cervical spondylosis, vestibulitis, autonomic neuropathy etc. Get your blood pressure checked in both supine and sitting position. MRI cervical spine is diagnostic for cervical spondylosis. Vestibulitis can be managed by sedatives and anti histaminics.
What drugs are you taking for hypertension?
The chiropractor does know about it, he can see it when I sit up at the table- I'm like a wobbly rag doll. I do take my time getting up. I've also got dizzy getting up from laying prone after getting massages from massage therapists in the past. Come to think of it, I even get dizzy after getting up from MRI's, though I don't remember getting a headache. I rarely get dizzy from laying at home because I'm always shifting positions. I will try drinking water beforehand next time..and maybe munching on some salty pretzels.
Does it only happen at the chiropractor or can you replicate the same symptom if you change position the same way at home? That would at least tell you if it's something the chiropractor is doing or if it's truly positional.
If it is positional, it could be a fluctuation in blood pressure from sitting up too fast. Can you try rolling over first and lying still like that for a few seconds to adjust, then *slowly* easing up and see if that doesn't help? Being really hydrated beforehand might help if that's it too--a salty snack and a bottle of water before your chiropractic session might do the trick.
If none of that makes a difference, perhaps it is worth mentioning to your doctor in case it is a symptom of something else. I would definitely mention it to the chiropractor, too, if you haven't already.