What Causes Raised Red Marks On Arms, Stomach And Groin Area?

I seem to have raised welts on the underside ( or Palm side up ) of my arms and on my stomach and groin area . They are like splotches and are itchy and red and burning. If this is some sort of allergic reaction how do I tell what caused it? And what is going on? I have no other symptoms. Is it from something I ate or in my environment? My doctor gave me a 5 day prednisone prescription with typical med and an antihistamine yesterday. Symptoms still there with intermittent acute flare ups. Help!