You're fine dude. If your gf is taking her pills according to the prescription, there is no time during the month that you could get her pregnant. See this page
http://www.plannedparenthood.org/bc/bcfacts11.html#PILL. They don't explicitly say it here, but I've read it on other websites too. Even during her "stop week" when she's having her period and not taking the pill (or taking the placebo), there is no risk of pregnancy even if you were to have sex.
I can personally attest to this as well. Only if she were to forget a whole day, should you use a condom for a week to be on the safe side.
So, absolutely no need to worry.