How likely is it to get pregnant on axithromicin and birth control.. is that an antibiotic that affects birth control?

Not really: I never make guarantees, but your chances of a problem are extremely low. Although some antibiotics can reduce the efficacy of birth control, Azithromycin isn't one of the known offenders. It's never a bad idea to use a back up method when you are on antibiotics, but i wouldn't be worried. If your period doesn't come on time, take a pregnancy test to be sure.