What are the chances a girl can get pregnant while on birth control and estrogen pills?

I have recently slept with this girl who happens to be a friend of mine. We did not use protection because she is allergic and I did not have any non latex condoms at my disposal at the time we were together. She is on birth control and showed me that she takes it religiously. Also, she is on some kind of extra estrogen pills. She told me that it is really hard for her to get pregnant even with out being on the birth control, and also said that since she has been on it, every guy she has been with has came inside her and she hasn't gotten pregnant. I am just wondering what are the odds that she can get pregnant while being on birth control and estrogen pills? She told me that the pills messes everything up "down there," I just want to know how high the odds are that she does end up pregnant. Thank you for any and all answers with advice or opinions!