Incorrect use of birth control and unprotected sex

I am 19. I have never been good at taking the pill, I am actually really bad at it, I forget it frequently and I think its definately about time I do something about it. I had unprotected sex on the Friday. I was due on my period in the next couple of days anyway and by the Sunday I came on my period. It's a normal period, but I don't really understand.. if I am on my period does that mean that I can't be pregnant or the fact that I had unprotected sex such a short time before may mean that it hasn't fertilised and stopped my periods or whatever? I really don't understand.
Thanks for any help.