Why is ma upper part of my arm Redish with little pimples, and when you pass my elbow its my normal tan arm?

Like really why are both of my arms the same , up my elbow its redish with little pimples that arent real pimples that you get on your face, they are like idont know but its weird and when it gets to my shoulder its my normal Shoulder with no red and no pimples!? its really scary it looks like sun burn because when i touch it it goes to white and returns to red!! but it doesnt huirt or anything!! please Help!! ( : thnk you