Sir, I Have Sex With My Girldrien Most Of Weeks

Sir, I have sex with my girldrien most of weeks ..I use condoms too,only 1-3time I didnt use cindom with her and she is virgin, I had sex with other girl more that one year ago....now for some week I saw some black marks in my dick head...I am afraid ,me and my girl talk about it..she it can be because of her teeth while oral sex,so is that common mark ??