Why when i walk on hot weather days my buttock begin to have some pink raised little bumps, and when the day pass is gone, but it return when rubb ski?

Heat rash: Similar to a common heat rash that an infant gets, any person can get a "heat dermatitis" from a hot, sweaty environment with skin irritation. These types of dermatitis are often self limiting and can resolve on their own when the skin returns to a normal environment. Skin that is prone to this is sensitive, like that of your buttocks, which may be why simply rubbing it brings it on too.