this is quite common. it's most likely due to an allergic-like reaction during exercise. during exercise histamines are released and some people are sensitive to them. people sensitive to histamines can experience rashes itching and other allergic symptoms during exercise. most of the time the rashes appear on the chest and thighs but can be on the arms or neck too. sometimes exercise also reacts with food allergies or medications (e. g. antibiotics pain medications sedatives tranquilizers and diuretics) to cause the problem. if the problem is bothering you so much that you can't stand it and it doesn't go away during the exercise session (it may just pass if you continue) then you should ask your doctor about using a topical ointment to stop the itching or taking an over-the-counter anti-histamine (like benadryl) 30-60 minutes before exercising. if over-the-counter anti-histamines make you sleepy your doctor can prescribe a non-sedative anti-histamine. you should only take over-the-counter or prescription medication for this problem after speaking with your doctor.
what does it mean when you exercise and sweat and then begin to itch?