Trembling or shaking can have many causes. Sometimes it appears in older dogs without any other discernable cause, however, if your dog’s appetite is also affected, it suggests there is some underlying problem. Causes can include weakness, joint disease, neurological problems, pain, fever, cold, after surgery, low blood sugar, emotional distress, metabolic abnormalities like vomiting, or shock. Your vet will take a thorough history of the problem and do a full physical examination. Further testing may be needed to arrive at a diagnosis.