hmmm.... it sounds like orthostatic hypotension. basically when you smoke weed it does some weird shit to your system and when you stand up from a sitting position and move around a bit your blood pressure drops phenominally as gravity draws blood out of your upper body and you feel dizzy and sometimes pass out, commonly known as "head rush" but if you've had seizures before you might wanna get it checked out man, can never be too safe.
chazz