Why do red blotches appear all over my body when i become hot or start to sweat?

Vasodilation: When you become hot or start to sweat, your body's blood vessels dilate (widen) in order to release heat and cool you down. This increased blood flow to the skin can cause red blotches to appear.

Histamine: Another factor that can contribute to red blotches during exercise or heat exposure is the release of histamine from the body's mast cells. Histamine is a chemical that is involved in the body's inflammatory response, and it can cause blood vessels to dilate and become more visible.

Dermatographic urticaria: Also known as "skin writing" or "scratch hives", this is a condition in which red blotches or hives appear on the skin in response to being scratched, rubbed, or pressed. It is thought to be caused by an overactive immune response, and it can be triggered by heat, sweating, or other factors that cause the skin to become irritated.