I am a 30 year old female. Non smoker or drinker. Im 5 7 and weigh 145 lbs. I have been on the pill since March when I had my yearly done and all lab work and pap smear results were good. In mid July I was diagnosed with a sinus infection and put on Zithromax (3 day) pack. Me and my husband used the pull out method. I got my period on August 4, which was about 3 days late and my period only lasted 3 days and it was unusally light. Now 16 days later, I am lightly bleeding again. Also, usually my breast get very tender about 2 weeks before I start and they got sore the day of the 4th when I started and have been sore every day since. I have taken several pregnancy test and they ve all been negative, so I was just wondering if the antibiotic could have messed up my hormones causing my periods to be out of whack for a month or so.