If it was a prescription allergy medication, your doctor should have asked you if you were on any other medications. If you told them birth control pills, the doctor should have told you if it decreased effectiveness.
I had to take penicillin for something a while back and my doctor told me to use condoms or refrain from sex.
I also got preg on birth control pills while taking penicillin tablets. But I took 4 a day for 5 days.
A lot of antibiotics cause birth control to be ineffective. You should use protection(condoms) until you finish the medication
I'm not sure how one pill will affect you, but I got pregnant while taking birth control and antibiotics!!