I think it's due to a lack of oxygen to the brain. I get that all the time when i smoke and then stand up...I used to like it 'cause i could just stabilize myself against the wall or sit on the bed when it happened but when i blacked out and fell against a table and cut my shoulder I decided it wasn't cool and now i take time to stand up more slowly. But i dont think you said you stood up, just that things went black, so i dont think you have to worry about falling. Unless it's scaring you, I wouldn't stress over it.