Hello,
It can be due to an allergic reaction to your soap or body wash as it is happening only after bathing. Use mild soaps or body wash which should be hypoallergenic. You can apply some calamine lotion at the rash as it will help in soothing the skin. You can take some OTC oral antihistamine medications like Benadryl or Claritin and see if it helps.
If the symptoms persist then get it evaluated from a dermatologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Allergy. Change your soap and shampoo.