[byuu]

I have that problem, and I don't exercise at all. Happens probably once or twice a week. Almost always from going from sitting to walking too quickly.

Usually, as long as I grab onto something and stay still, it passes after about 20-30 seconds of my vision blacking out and my thoughts getting all jumbled and incomprehensible.

I've only fully blacked out from it once, coming to and realizing I was twitching on the floor.

I've been led to believe that it's a fairly common condition, though.