Why always when rub my buttocks and sweats , it makes a itchy rash, but when i wash its gones away , but always happens, how to prevents, or what is?

Rash: It appears you develop an Irritant Contact Dermatitis which results from the prolonged contact of the sweat with your skin. The main thing to prevent this is to try to stay as dry as possible and if you notice you are starting to develop moisture in the affected area, wash immediately. In a moist enviornment, a fungus like to thrive, so visit a Dermatologist if symptoms persist.

Stop rubbing: Your problem may well be from dermatographism (oversensitive skin) which is harmless by itself. Try taking an antihistamine to see if it helps. Dry skin can also cause it and thus applying a moisturizer after bathing may also be helpful.