certain antibiotics cancel out your birth control, you should always use a back up method when taking antibiotics. the doctor that prescribed them should have told you that. being on your period though it is unlikely, but anything is possible.
it is possible to get pregnant at anytime... just let me tell you, dear, that a lot of antibiotics do cancel out your birth control... so pleas, hun, be really careful.
the chances a very very low ,but you have to wait a take hpt to confirm. Good uck