This sounds like a form of Physical Urticaria called Cholinergic Urticaria which is triggered by sweating, exercising and showering. Its not usually caused by an allergy. However keeping the skin soft and well moisturised and avoiding the physical triggers will help. It does have a habit of suddenly flaring up and stress can play a role. If Benadryl antihistamines don't help, a low dose beta-blocker such as propranolol could be tried.
The information provided does not constitute a diagnosis of your condition. You should consult a medical practitioner or other appropriate health care professional for a physical exmanication, diagnosis and formal
advice. Health24 and the expert accept no responsibility or liability for any damage or personal harm you may suffer resulting from making use of this content.