My god dude, quit playing with fire!
There is a chance, so don't risk it. BUY THE GOD DAMN RUBBERS.
If you want to play like a man, act like one.
Trust me this is the voice of experience.
It can happen, but it's very unlikely. A woman ovulates (releases an eg) about 14 days after the first day of her period. So...by the time her period comes she should not be able to get pregnant. Good luck, and next time be careful or just do oral.