What Causes Itchy Red Spots Under Breasts And Arms?

I have a rash on my body that is red spots from head to toe. The ones that are on my feet between my toes itch and are breaking the skin between my toes. They are under my breasts and under arms. back, both arms, and legs and buttocks. This started about a week ago and I have RA, Fibromyalgia, and IBS. It's also in my mouth on lower lip and inside both cheeks, and back of throat.