Do you have any rash at all on the affected areas? scaling skin?
Are you sure this all started after the sexual encounter?
Any evidence of pubic lice?
Do you happen to know if your partner has any STD?
Can you post a picture?- I do not know what you mean by a "couple dark small rashes"?
Do you actually have any lesions now? Or just when it gets moist?
Also- I see you have posted 3 questions but not 'accepted' any answers.
Please be aware that I will not respond further if your intention is not to "accept" a helpful answer.
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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POSITIVE feedback & a BONUS are warmly appreciated. Please note that answers are for information only, do not take the place of an IN PERSON assessment by your doctor,and does not establish a patient-physician relationship.