It can be due to exercise induced urticaria. Most symptoms can be controlled by taking antihistaminics like benadryl or Claritin or zyrtec. Slow down or stop your exercise as soon as symptoms start. You should always exercise with a partner who knows about your condition.
In some people, eating certain foods before exercise may make allergic symptoms more likely to occur. Keep track of what you eat before exercising for a few weeks. If you notice a pattern to your symptoms that seems related to a certain food, stop eating it for a while and see if the hives stop.Also, do not exercise for 4 to 6 hours after you eat.
You should stop exercising as soon as you notice the hives. If the hives don't go away in 5 to 10 minutes, or if you have other symptoms, call your doctor right away. It can be Exercise-induced anaphylaxis.
Also wear medical alert jewelry or ID tags describing the condition and sensitive substances and always carry a self-injectable epinephrine kit (Epi pen).
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.
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