your dr may be thinking staph folliculitis .if you havent tested for stds i would tend to agree with shaun .you might want to test for stds .you might want to have the rash biopsied so you can treat it .good luck.
You need to get tested for a variety of STDs. It could be anything.
At minimum get a Syphilis test and to be safe you should get an HIV test -- at least 4 weeks after your last unprotected sexual exposure. HIV test should be 4th generation (or at least 3rd generation).
The Syphilis test also take time after your last exposure. But if the bump is a chancer, then it can take the conventional Syphilis test 4 - 6 weeks after this appears to show that you have it. Discuss what test you need with your MD. If it's been less time, you can ask for a PCR Syphilis test.
Be well.