My dog Nix was doing the same thing. So I called the vet. He had red lumps on his body all over and he was scratching them until they bled and scabbed over. My vet overreacts to everything, but I still usually bring my dog in when she suggests it.
In the end, we had a mite problem. Not regular mites. Some sort of rabbit mite that rarely infects dogs. It was a crazy long, expensive process to solve. But Nix is furry and happy again.
Is you dog on any flea/heartworm stuff? I had to switch to Revolution because Revolution kills mites but Heartguard doesn't.