Hello,
It can be due to solar urticaria, sweat dermatitis,
heat rash or contact dermatitis. I suggest you to consult a dermatologist and get it evaluated. Till that time, you can take some OTC oral antihistamine medications like Benadryl or Claritin and see if it helps.
For mild-moderate symptoms a weak steroid may be used (e.g. hydrocortisone as dermacort), whilst more severe cases require a higher-potency steroid (e.g. clobetasol propionate, fluocinonide).