I think what happiness03 was trying to say was that even before you ejaculate there is precum which does have sperm in it.
So, since birth control isn't 100% effective, and there is precum before you start using a condom, there is an extremely small chance that she could get pregnant. But that is extremely unlikely since she is on birth control.
I had used birth control pills for 3 years and we did not use condoms, and I will admit that I didn't take it 100% correctly. There were some days I would take it an hour or so late, sometimes even six hours later. But I never got pregnant.
I would honestly not worry about it if I were you. If you think about it, you aren't using condoms all the time, so what good are they really doing?