Why did I blackout yesterday.?

Im 14 and a vegetarian and been one for about 3 years. Im 5'0 and about 95 pounds.The other day i was climbing down the stairs at summer school when all of a sudden i couldnt hear or see anything. I woke up at the bottom of the stairs and people said i had fainted/blacked out. I thought this was weird because i had lunch and i drank alot of water. I didnt feel dizzy before or after i fainted/blacked out. So can anyone tell me what possibly happened..?