It sounds like you just hit your head too hard on the fall, and falling asleep after a concussion is definitely not smart you should get it checked out. I'm sure it is just related to being light headed.

Your situation sounds very similar to a friend of mine . We used to smoke regularly and after getting high dozens of times he seemed alright. Until one night we were smoking and he went inside with me and had never stood up abruptly but ended up fainting and hitting his head. It happened to him every time after that (3 more times) and he just quit smoking. We never did get to figure out what was the cause because he doesn't smoke anymore after that, so I really don't know what to tell you.