What Causes Redness On Neck Radiating Towards Shoulders And Chest?

my neck is (very often) red, not at all itchy, but sometimes quite warm- resembling a heat rash. it also sometimes continues to the top of my shoulders and upper chest and is most prevalent when i m warm. i m 19 years old and extremely healthy and on no medication. what is the cause and how do i stop it from happening? mainly for aesthetic reasons...