On Friday Morning I Decided To Take An Antibiotic After

On Friday morning i decided to take an antibiotic after feeling as if I had a uti. I only took the one pill because I forgot to continue with the antibiotics. I am on birth control and have always been extremely reliable with my birth control everyday. I know that antibiotics affect birth control, but it was only the one pill 3 days prior to my problem. Here comes the problem: I had intercourse on Sunday night around 11:45 and he finished inside of me. Should I get plan b or do you think I'm fine?