I was on birth control last week, before my period, and hadunprotected sex four days after starting antibiotics. it should have been my second day of period, but i am just spotting.what should i do?

Not likely/possible: Antibiotics can reduce the effectiveness of bcpills and it is recommended to use another birth control method if you are taking antibiotics, i will suggest you do a pregnancy test and that will decide if your period was normal or missed.

Pregnancy test: You should do a pregnancy test just to make sure that you aren't pregnant. Antibiotics throw your system off and it could just be a simple thing like that, but you should make sure you're not pregnant.