Please help

Patient: Hi i m on birth control for 6 years and my period is regular,i had to take antibiotics for a bad sore throat,some people say it can take the effect of the birth control pill away so i m a little scary. i took my regular pilll every day at the same time but last friday night we had unprotected sex saturday morning i took my regular pill and bought the plan b pill.my period was suppost to came today and it hasn t so i m a little worry

Doctor: