The symptom you are describing is called orthostatic hypotension. It is the failure of the cardiovascular system to adjust to changes in body position. Normally when someone stands up, this results in a sudden fall in blood pressure in the head. This triggers an almost instantaneous reflex where blood vessels constrict to maintain blood perfusion. Orthostatic hypotension is a failure of this mechanism resulting in lower brain perfusion which causes us to pass out, or feel like we're going to pass out. This mechanism can fail in patients who are severely dehydrated, have severe atherosclerosis, or those who have autonomic nervous system (ANS) dysfunction (mechanism for the reflex). Patients with orthostatic hypotension will not necessarily have a low blood pressure, it will just drop inappropriately when standing. The question of whether HIV is the cause of your orthostatic hypotension is difficult to say. There is some preliminary evidence that HIV infection can cause autonomic nervous system dysfunction. However, more studies will need to be done work out whether this is true or not. In the mean time, you should see a doctor
about your symptoms. I would schedule and appointment with your primary care physician
who can perform in office orthostatic tests. Certain cases can be effectively treated with aggressive hydration and steroids. Good luck.