I faint all the time. I'm very prone to hypoglycæmia and low BP, and I also suffer from orthostatic hypotension (extreme dro in blood pressure upon sitting or standing up). It's fairly common in young people to suffer from syncope upon elevation, or due to dip in blood sugar because of fluctuations in the metabolism. Keep yourself well hydrated, and make sure you're eating properly, and you should be fine.
However, any loss of consciousness (even for a few seconds) is significant, and could indicate an underlying problem. It is always important therefore to go to the doctor and ask him or her to check you out.