The short answer is YES, the only TRUE 100% fool proof method to prevent pregnancy is abstinence. Anytime you have sex, even when you use a barrier method, pull out method or other form of birth control there is a risk, however small, of pregnancy. But like spade22 said it is highly unlikely that she is.
Its highly unlikely but remotely possible since birth control has a small failure rate and a women can get pregnant at anytime during her cycle (if she ovulates during that timeframe).