Since you were regularly taking the birth control pills and have started with the antibiotics after 3 days after intercourse, chances of the effectivenesss of birth control pills being effected and pregnancy are very less likely.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.