Number of reasons:
MOST likely something called POSTURAL HYPOTENSION -- characterized by a faint/dizzy feeling when standing from sitting, or from lying down to coming upright. If it was low blood sugar, it would happen without regard for your posture. There may be an anxiety component in that if you are afraid you'll feel faint especially during exercise, it will magnify any blood pressure issues (not that you need treatment for anxiety, just saying it doesn't help, that's all).
You should see your MD about it. If it is a mild case, then no treatment is generally required. But if it is severe, the underlying causes should be identified. It could be as simple as dehydration.
Last edited by kiramira; 07-26-2009 at 02:12 PM.