Birth control is likely to work , she doesn't have much work to do :D ..
Pre-c*m can contain sperm but not much as full ejaculation ..
Relax yourself
*Add bit info for thatquietgirl's reply*
Some (but not all) antibiotics do lower the efficacy of birth control. If the doctor told you to use a back-up method, you should have listened. Pre-ejaculate can contain some sperm--specifically if he has ejaculated beforehand and not done anything to clear the urethra. If your antibiotics lowered the efficacy of your birth control pill, you very well could ovulate while on your pill.
Normal use as instructed without anything interfering with efficacy is believed to result in three things that work together to prevent pregnancy: 1) uterine lining thins to prevent implantation, 2) cervical mucus thickens to prevent sperm from getting inside, and 3) ovulation does not happen because your body's hormone levels are kept steady rather than experiencing the ebb and flow of a normal cycle.