Hello --
The chances of this being an STD are fairly low, although it could possibly be an orally transmitted form of herpes. I don't think so, though, based on the one picture that's been allowed on the site you used. From that remaining picture this looks like some sort of contact
dermatitis, and if so could be due to overuse of soap, a change in soaps, or something like use of fabric softener in laundering of underwear. Just a guess, and the picture isn't real sharp or clear, but at least does give a general impression of what's going on. Based on that I think probably a
dermatitis, contact type, which might respond to a topical steroid cream your doctor could prescribe, and your doctor definitely should be consulted to make sure this is an innocent rash, which it appears to be.
I hope this helps. Good luck to you and please follow up as needed, here.