If you do not have any visible lesions now, then I doubt this is an STD.
It may possibly be a fungal infection- which can be treated with a topical OTC antifungal ( like nystatin or Lotromin), or it may just be a form of eczema or dermatitis.
It sounds like the first step would be to minimize moisture and optimize hygiene.
I would also try the antifungal therapy.
If it does not respond you may well want to see a dermatologist. I would be hard pressed to advise anything else without a physician seeing it in person and examining you.
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