Could be everything tiggersmom said but it could also be a food allergy. My rottweiler had a so called hot spot on his hind end at the base of his tail. We took him to the vet and ruled out mange, infection, insect bite and any other parasitic thing known to dogs. We spent a few hundred dollars and found out he was allergic to beef by products of all things. Put him on a beef free diet and a month later the hot spot was gone and his coat grew back in. Just some food for thought. He was much happier too.
