Hi there ~
I understand your concerns and if you have been tested for STIs and there is nothing concerning in those reports, I advise you to consult a dermatologist
. Even if it is an allergic rash
you need to know what the allergen is and if changing your detergent used to clean your garments also is not helping, a consult with a dermatologist would help explain and treat your condition after a physical examination of the spots.
I hope this helps.
Take care and have a lovely day!