it is called VASOVAGAL SYNCOPE - it's hypoxia from less than 16% O^2.
http://en.wikipedia.org/wiki/Fainting#Vasovagal_syncope
It drops your heart rate, your blood pressure, reducing blood flow to the brain, and you lose consciousness. You don't have control over it - you just feel the dizzy or hot, light-headed, and next thing you know you're waking up after being unconscious for a few seconds. It's a really interesting process...something about the neocortex orchestrating this all...this mechanism also works to make some people faint from blood, spiders, etc. Except in those people they're not having problems with oxygen. It's weird. I have passed out like this 3 times - two times from smoking and once from being sucker punched in the chest.
I've been here a couple times, one was contributed to weed another mushrooms and three times just because I guess. My Hypoglycemia might have something to do with it. The best thing I've done to prevent this is eat lots of fruit and bread, snickers bar in a pinch. Keep hydrated also.