You are correct that anti biotics can lower the effectiveness of birth control pills (some more than others). Unfortunately many people don't realize this and their pharmacists fail to tell them. Yes, it is possible to become pregnant while having your period. Not likely, but possible. My advice is when in doubt, be cautious. It's difficult to answer your question exactly without knowing what antibiotic you were taking. I suggest asking your pharmacist and be careful until you know for sure.