What Causes Itchy, Red Rashes On Legs And Arm?

Male age 48, 6 ft. tall 300 lbs. Have a rash that is red spots that are like welts and are different sizes and are scaly. It started on my legs, now all over back and legs and starting on arms and chest. It itches and if i scratch it a layer of skin comes off and it bleeds and it burns. What could it be. My doctor can't even tell me what it is.