It itches all over when it's hot.

Hello! According to your symptoms, your condition belongs to artificial urticaria, which is common in young people with allergies. Most of them are allergic reactions after the skin is stimulated by external physical stimuli.

Suggestion:

This disease generally does not need treatment and will get better with age. Generally, it is necessary to remove the factors that can make the skin allergic, and antihistamines, calcium agents and hormone preparations can also be used to control the disease. I hope my answer can help you!