Could I be pregnant?

Juliette
Okay so I've been sexually active and I'm not on birth control. However, we almost always use condoms and then occasionally he pulls out. I bled the day before I was suppose to get my period but then stopped and there was no more blood other then that ONE day. This has never happened to me so I was thinking that I could be pregnant so I took 3 pregnancy tests (on different days) and they all came out negative. 
Is it possible that I got my period for just one day? (It wasn't even that much blood, just enough for one tampon)
Has this ever happened to anyone before and what do you suggest I do?