Sex on period

My boyfriend and I generally don't have sex while I'm on my period. Yesterday I told him it was heavy, since it was the first day, I didn't think he would want to have sex because we never did in the past. I don't mind, it's just very messy. 
Also we've been useing thewithdral method for a year now and things have been fine. We are careful about timing and such. 
However, yesterday, for the first time he came in me while I was on my period. I think my chances of pregnancy are slim. We are not planning a pregnancy, but if it happens its welcome. Thaughts?