I agree w/ your doctor and mwolf1428: most likely orthostatic or postural hypotension which may be related to dehydration, certain medications (including blood pressure medications and diuretics among others), or simply just changing positions too quickly. Other things to consider: cardiac conditions, vertigo, or meniere's disease. Keep your doc updated on your sx and good luck.
It could be a condition called orthostatic hypotension which is basically abnormally low blood pressure that occurs when an individual suddenly assumes the standing posture. It can produce dizziness and fainting. Also called postural hypotension. While I am unsure as how a doctor would go about monitoring this, it is a suggestion that you could take to your next doctor. I would suggest gradually coming to the standing position allowing your arterial and venous pressure to equalize before you are fully standing. Good luck!
Your glucose might drop when you stand up quick I would have a glucose test. It could be your dehydrated or your blood pressure is dropping.....