What Causes Bright Red Rashes On Inner Arms And Legs?

My husband developed a bright red rash on his inner arms and all-around his upper legs and down the back of his calves. This started two days ago. It is warm to the touch. It looks like sunburn (but it is not). It is slightly itchy. It is raised slightly at the edges but is in big areas that cover the entire surface (not blotchy or lacy). What could this be? He did have a fever a week or two ago.