I have been on ortho novum 777-28 birth control for a year now, but I started my set of pills at the beginning of April a few days late. I took all of April's pills daily around the same time. My week for my "period" was May 4-10. The day I got off my "period" I had sex but the condom broke and my boyfriend is unsure if he pulled out in time. This was on May 10 and if I were to ovulate it should be around May 18-20. Also, I took antibiotics May 1-3. Would the antibiotics(omnicef) prevent my birth control from working? Or would the starting my birth control late the month before have an effect?