Hi
Welcome to the forum!
This could be due to pressure urticaria due to stinging of water spray. If you take a tub bath or a bath with bucket or mug you may not have this.
It is difficult to comment beyond this at this stage. Please let me know if there is any thing else and do keep me posted. Take care!
You may have "physical" or Cholinergic urticaria". Any rash (hives) appear together with itching? Next time when itching start take an antihistamine pill (like Benadryl). If this will help you, it means you have physical urticaria (allergy to heat in your case).