You can get pregnant any time. You don't need a period to ovulate, you need to ovulate to have a period.
Hormonal BC's for the most part have a 99 percent success rate.
That is why someone can get pregnant before they even get their period.
My fell out and i wouldnt of had any idea it happened if DH and I hadnt been messing around. He found it and was like WTF is this lol All i could say was "smurf, that's my birth control!" It was a mood killer