You need to see another vet. This happen to my GSD almost two years ago and my vet did x-rays. Found out he had arthritis in the ctr. of his back. Got to the point where he would constantly pace and really had a terribly hard time trying to lay down or get up. Pain pills only helped so long. This may not be your dogs problem but there IS something wrong and a new vet is definitely in order. Your dog is in pain and you need to find out why.