What you’ve described is likely to be urticaria (or hives), induced by exercise/sweating/heat in this instance. It can occur at any age.
The itchy rash tends to appear on your front, back, and chest, as you’ve described. I believe that the colloquial term for this is “feng mo”.
Urticaria/hives is very common, and usually goes away on its own. If it persists after your exercise, you can see a doctor to prescribe anti-histamine tablets, which are very helpful in getting rid of the itch and rash.
Anti-histamines have minimal side effects, and I always keep a stash of them at home myself.
You could also try taking antihistamines an hour before exercising, which may help to prevent the rash from coming on.
It’s also useful to wear loose, cotton clothes to allow your sweat to evaporate.
If all of these measures do not help, your doctor may consider a fungal scrape test after examining your rash. Occasionally, rashes that appear at skin crease/sweat areas can be due to fungus.
Snap a photo the next time you get this rash! It will be very helpful for your doctor to confirm the diagnosis.
The good news is that hives triggered by exercise tends to improve with time, and many patients find that the problem goes away by itself after about 2 – 3 years.
