I was prescribed clarithomycin for a sinus infection and was warned it coukd interrupt my cycle, however I m on the pill, and my bf and I used condoms while I was on the pill. My period is now almost 4 days late... could this be from the antibiotics? My bf and I don t normally use condoms as I have a slight allergy to them, and had a drunken night shortly after I stopped taking the antibiotics... I m very faithful with my pill, but could I be pregnant?