What Causes Severe Skin Rashes After Taking Sulfasalazine For Ulcerative Colitis?

I was diagnosed with ulcerative colitis and prescribed sulfasalazine. Soon after starting this drug I developed a bad skin rash which partially cleared within a couple of weeks. At this point I became very ill with muscle and bone joint aches starting in the feet (numbness and tingling) and going up through the back of the legs into both sides of my back. I had severe chills with no fever. The joint/muscle pain then extended to my neck and shoulders. I went to the doctors with this and was provided a pain shot and prednizone. The problems cleared up with the prednisone but came back when the regimen was completed. My doctor may know what the problem is but has not told me. I asked for more of the prednisone but was denied and provided with Percocet s instead.