Chances are that you are not.
If you have a regular, 28 day cycle, you most likely already ovulated. Since there was only 4 days between the date you had sex and the date you were supposed to start your period, it isn't likely.
Next time you are on antibiotics, use a back up form of birth control.
That is how I became pregnant.
If your period has not arrived within the next few days, take a pregnancy test.