Sure, some antibiotics can interact with birth control pills and make them less effective and pregnancy more likely ... but the Mayo clinic says,

"Hypothetically speaking, other antibiotics, particularly penicillin and tetracycline derivatives, could impair the effectiveness of birth control pills. However, no large studies have proved such an effect.

Researchers can't rule out the possibility that a small percentage of women may experience decreased effectiveness of birth control pills while taking an antibiotic. And if you're taking a newer, extremely low-dose oral contraceptive, you could be more susceptible to these potential effects from antibiotics. If you're concerned, consider using a barrier method of contraception for the duration of your antibiotic prescription." http://www.mayoclinic.com/health/birth-con trol-pill/WO00098

What type of antibiotic are you taking?