Hi,
I can understand your concern about the rash in view of your exposure history. Certainly, some STDs can cause rash/ skin lesions like Syphilis, Herpes and HIV. Typically, secondary syphilis (that means syphilis that has not been treated and the disease progress to 2nd stage) can cause a rash but the rash is usually not itchy and usually appear as small bumps.
Herpes usually cause blister-like skin lesions. HIV rash can vary in appearance and there is no specific characteristic rash that can definitely indicate that it is a HIV rash.
Looking at your photos, the rash looks like it could be either a allergic reaction (ie. hives) or fungal. Both of these conditions can cause itchiness as well. However, I would recommend that you visit a doctor for a thorough assessment in order to determine the treatment.
