What Causes Painless Red Bump On My Penis?

I have a small red bump on the shaft of my penis, I have had unprotected oral sex with my girlfriend and she did have a small cold sore. I'm afraid it could be genital herpes but it doesn't look like what i see in pictures. It is a single red bump, and there doesn't appear to be any fluid present. I tried popping it but it wouldnt pop.