My arm itches when it's hot.

Hello, consider cholinergic urticaria. Cholinergic urticaria mainly occurs in young people and is induced after exercise, sweating, emotional tension, intake of hot food or alcoholic beverages. Wind masses with skin lesions of about 2 mm, with obvious redness and swelling around them, are distributed in the upper torso and upper limbs, and do not merge with each other. Sometimes there is only itching without skin lesions, which generally does not occur on the face and disappears after about half an hour to an hour.

Suggestion:

It is suggested to take cetirizine or loratadine or ketotifen orally and Piminxiao capsule orally. Good luck.