What Causes Rash On Legs,elbows And Knees?

My 8 year old son was diagnosed with molluscum a couple of months ago. Recently he developed a rash covering his arms and legs, severely on elbows and behind the knees. Dr. says she thinks it is an allergic reaction to the molluscum, and presribed Keflex and steroids. She says it isn't anything to worry about, that he's not at risk for auto-immune diseases...is that true? Should we be worried?