Sometimes I'll be walking around, doing my own thing, and suddenly my vision will go black for 2 to 3 seconds and I lose all sense of balance (I never actually pass out though). If I hold my breath for over three seconds while standing up, I black out again. I am hydrated and do regular exercise, and last time I checked, by blood pressure was normal. I'm an 18 year old male, if its of any relevance. What can be causing this?