YES. TWICE. and like you, i never found out why. although it's probably worth getting a blood test to make sure you're not anemic.

the last time was when i was walking back to a friend's place. i started to fall over, then everything turned green and i couldn't hear. i made it to her staircase after i could see again, then faceplanted and passed out for (according to her) 5 minutes. i thought i was going to die but then i woke up and had some ice cream.