What Causes Rash On Feet Spreading Towards Legs, Arms, Abdomen And Buttocks?

Hi I started with a small rash on my feet on Monday the 5th of January. And today 8 days later it has consumed my legs and has moved up to my abdomen and butt. It is now starting to break out on my arms. It is very painful ans I have a burning sensation all over my legs. Some spots have formed into bruises ans I also started getting muscle spasms on my legs. Please help I spent 17 hrs at the emergency last night and they didn't find out what it was . They said my labs came back fine and no medication helps with the pain. Thank you
Claudia Barco