Late & On Antibiotics---Pregnant?

i'm starting to worry that i might be pregnant, but wondering if being on antibiotics while i'm supposed to have my period could cause it to not come.

i've been on the pill for about 7 months. my partner and i ended up having unprotected sex the morning after i thought my period was over. i ended up bleeding the rest of that day and then it was done.

however, i took my last pill this month last thursday 7/27 and still haven't gotten my period as of today 7/31. i'm due to start my next pack on thursday. i also started taking an antibiotic (chlarithamycin) during my off week.

i've taken 3 pregnancy tests and they're all negative. should i really be worried?!?!!!!

hope someone can help