Antibiotics render birth control ineffective, so, yes you could certainly be pg. Take a test!
I use Answer brand and they're only about $8 for 2 tests.
I just went on antibiotics myself and am on Ortho Evra. I asked the pharmacist and she said that you only have to use backup contraception for 2 weeks (it was a 5 day dose) and that according to the research, antibiotics don't do too much to renderring your birth control ineffective, but that was probably just the kind of antibiotic I had. Definatly take a pregnancy test.