Shaking, loss of appetite and lethargy, all are symptoms of many diseases and conditions. Lethargy is suspected if dog have reduced activities, drowsiness, and delay in response to external stimuli. If your dog show these signs for more than 24 hours then visit to vet is necessary. Canine lethargy can be due to anemia, electrolyte imbalance, cardiovascular disorders, infections, hormonal disorders, cancer, skin and eye diseases, side effects of medicines, toxic reactions, and immune diseases and many more. So, It will be good to visit a vet for differential diagnosis and accurate treatment.