Most likely you have heat rash or a type of eczema.
This is due to the immune reaction against the skin.
Fungal infection is another common cause.
heat and sweat can promote fungal growth.
To address both the issues, you can use following over the counter remedies twice daily for 10 days:
1. Clotrimazole or Lotrimine cream
2. Hydrocortisone or Cortizone cream.
If no significant improvemen in next 3 days, that means you will need prescription strength medications.
You should see your doctor in that case.
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