Semen can live a long time for such small things, I think up to 3 days or somethings in a womb, so if I were you i'd get myself checked out to be sure, that is assuming you have had sex one day after menstration.
You are unlikely to get pregnant from sex on days you are bleeding, or for a day or two after bleeding stops. But it's not guaranteed, and it's not a risk worth taking.
You can get pregnant at anytime. Just because you had your period is meaningless. Periods are not a form of birth control.