What Could Itchy Rashes On Rib Cage, Abdomen And Shoulders Suggest?

I have a rash that started out itchin but nothing was really there and i would get scratch marks. Now its turned red and is spreading on my rib cage, abdomen, and now shoulders, the shoulder part feels a little like a sun burn, i have been itchy for a while but just noticed the red spreading. What could this be?