Thank you very much for the useful information.
This can be due to localized skin irritation and excoriation secondary to moisture (sweat), friction (from garments/intercourse) complicated by a fungal infection.
Right now you should keep the affected area clean and dry.
Avoid tight fitting clothings.
Use topical anti fungal ointment (available over the counter) such as Tinactin or Lotrimin.
You can also intermittently apply Lignocaine ointment for relief from irritation or burning.
See how you respond to these measures. Most likely this is going to settle down soon, but if this does not improve, persist or get worse, you would need a clinical examination by the doctor and testing for STD's as well.
I would have been worried too if I were at your place, but let me reassure you that this does not sound serious at all. It is a treatable condition and carries a good prognosis with appropriate management.
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