All I can say is it doesn't sound like an STD, including herpes. That's my only area of expertise, not dermatology or skin conditions. Scabies sounds possible; it is a
commonCommon cold cause of a bumpy rash around the buttocks, legs, waist, and genitals, and often is intensely itchy. Scabies often is acquired during sex with an infected partner, or sharing his/her bed; and 3 weeks is about the right time for onset of symptoms. But it could be folliculitis (sort of a form of acne), fungal infection, or something else entirely.
I can't think of any profession that requires you delay seeing a healthcare provider; that's what you need to do. Not knowing what it is, I can't recommend any particular OTC treatment. If you want another online opinion, you might post your question to Dr. Rockoff on the Med Help dermatology forum--but I suspect he will also say you need to see a provider and not try to guess at treatment.
Good luck-- HHH, MD
HHH, MD