First time taking a plan b pill

Okay so I’m sick right now and I have medication that I need to take to get better but I’m scared to take it because I had unprotected sex last night and he came on my butt then the cum dropped down to my vagina and it was a lot of cum. So we went to go get a plan b and I read the back of the box and it says do not take certain medications because it can be less effective. Idk what to do