that used to happen to me alot when i was I'm thinking between 12 and 15, i would get light headed and just black out, then it just stopped, never had it again in the last 35 years ( well a few times i did but that was booze related lol )
my parents ran alot of tests on me thinking it might have been the start of epilepsy or what not, but the Doc's never did find anything, i recomend to keep a diary of the occurances and keep your parents informed, then they can go from there