Itchy rash that started 3 weeks ago all over stomach chest and back?

Rash: The differential of an itchy rash is very large. Try to describe it. Is it red and scaly? Flat or raised with fluid filled blisters? Is it scaly? In your body folds? Itchy rashes have several causes, urticaria, or hives, are very common and usually indicate an allergic reaction. A picture is worth a thousand words when discussing a rash. Know the timing of onset and the evolution of the rash.