I brake out on the back of my legs why?

Acne. Pimples originate in pores in the skin. Pores contain cells that produce hair and glands that secrete oil called sebum. The body is covered in skin pores that are susceptible to becoming clogged with dirt, oil and bacteria, which causes pimples. The national institutes of health indicates that even though acne is most common on the face, it also can occur on legs.
Need to be examined. Rash on the back of your leg requires a history and exam. Possibly contact dermatitis.