Should i take a plan b?

I am currently on an antibiotic and am at the end of the treatment in a few days, on Thursday I had sex and we used a condom because the antibiotic makes me birth control less effective. My bf finished in the condom and then he got dressed and after a little bit asked if I wanted to go ago. so he started without a condom and put a condom on so he could finish. I should mention this all happened on the last day of my period. My only worry is the chance residue sperm has gotten in me. Would it be worth it to take a plan B or does it sound like I should be okay? Thanks!