What Causes Rash Around Groin, Chest, Neck And Legs?

My 9 year old has had a rash on him (no fever) for about a week. It started around his groin area and progressed up his chest, neck and down his back/legs. He was treated for strep (which his strep test was negative) but after a week, the rash is still here AND he has developed small knots in the crease of his leg as well as a few in the back of his head. Any idea what it could be causing this??