Anytime you have unprotected sex, even on birth control, you can become pregnant. Birth control states that it is not 100% effective at preventing pregnancy. I've known more women who've gotten pregnant on the Pill than women who've gotten pregnant on purpose.
You can also become pregnant on your period, depending on your cycle length. Your period does not determine when you ovulate. Ovulation occurs based on hormonal fluctuations in your body. This means that even if you project you'll ovulate at a certain time each month you could ovulate sooner or later than suspected. Women ovulate about 2 weeks before their period and sperm can live 5-7 days in the vagina.
I was on birth control for about 4 or 5 years and had unprotected sex VERY VERY VERY OFTEN during that time. I never became pregnant. I would say the chances of you becoming pregnant are very slim. Also, its even harder to become pregnant while on your period.....not to say that it's impossible (because anything is possible) but this is even more unlikely . You should be ok, but just in case, pick up an at home pregnancy test around the time of next months expected period to be sure. good luck =D
due to the fact that you are on bc and you were on your cycle at the time the chances of you becoming pregnant from that one incident are almost zero