Thanks for writing to DoctorSpring.com with your health concern.
Yes, penicillin will interfere with the BC, reducing its efficacy.
You have taken Plan B, so do not worry, you stay protected against pregnancy.
However, if you are going to be sexually active for the rest of the month, then use a backup method like condoms, before you start a new pack.
Hope this helps, would be glad to answer any follow-up queries.