Red irritation on penis head

I haven't had sex in about a year, however about six months ago a red mark on the rim of my penis appeared. It was about the size of eraser head maybe smaller, it didn't hurt or puss. It has remained and after masturbation seems to be a little more noticeable but never hurts. The only discomfort seems to come sometimes if I walk a lot my penis head seems to be irritated from my boxers, but not in on that spot. I may just notice this more because I think about that mark. I was in a committed relationship prior to this mark and unless my partner lied she said she was std free. I was wondering if I could have picked something up some other way if this sounds like an std.