Hello,
I cannot confirm anything without examination but it can be sweat dermatitis, heat rash or hives. You can apply calamine lotion to the affected area. Oral antihistamine, such as Benadryl can also be used. You should take a comfortably cool bath and wear smooth-textured cotton clothing. You should choose mild soaps without dyes or perfumes and use a mild, unscented laundry detergent when washing clothes, towels and bedding.
Please take a second opinion from a dermatologist if the symptoms persist. I hope it helps. Take care and please do keep me posted on how you are doing or in case you have any additional doubts. Kind regards.
Extra Info i forgot to include, this tends to happen when i masturbate