should i take a plan b?

I will start by saying I had a similar encounter about 2 weeks ago and took plan B which I have had a period from already. I was on the last day of my period when this happened. 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. 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!