The pill and antibacterial tablets = pregnant?

Please feel free to move this if needed.

My girlfriend has been on the pill (Microgynon) for 2 months now. She should have started her period yesterday but hasnt (She has all the feel as if she is on just no blood) during last month the doctor prescribed an antibacterial tablet (not an antibiotic) for her to take once a day for 5 days. We asked if this would effect the pill's effectiveness the doctor assured us it wouldnt and there was no extra reason other than the normal ones to use extra protection such as a barrier method.

Well my girlfriend is now a day late on her period and is nerveous she is pregnant. What are the chances of the antibacterial tablet effecting the pill?