Diagnosis, especially in dermatology, cannot be made without at least seeing the lesion (not to mention biopsying it). Personally, I think that if the Betamethasone hasn't worked that you could be dealing with an infection, not necessarily an STD but probably so, given your past exposure. The solution is to ask for a professional opinion at a real clinic, not the internet.
You need to see a doctor, especially since the meds are not working. Also, really be careful and have protected sex, one can not be too careful this day and age.