What Is The Treatment For Dark Patches In The Legs And Arms?

Hi, Im only 17 but have had these darkened spots appeared about 3 years ago on my upper legs which have now completly spread to my lower legs, arms, and trunk. They some times turn bright red normally after doing exercise. Possibly due to heat change or something? I have seen my GP and they have no idea. My bloods have came back clear. What could this possiby be? Help!