The Last Time I Had Sex With My Boyfriend, Well

The last time I had sex with my boyfriend, well now ex-boyfriend, was two weeks before my last period. My last period was a month ago. I got my period two weeks after we had sex and we did use a condom. I don't know why I am fearful of being pregnant because I did get my period during a time period where getting pregnant is the most likely while using a condom, especially unprotected.. but I also have been taking antibiotics recently because I recently had surgery in my mouth for a tooth implant. So are the antibiotics causing a little delay in my period?