The symptoms you've described, including shivering, lethargy, and a normal appetite, could be caused by a variety of factors, some serious and some minor.
It's crucial to contact a veterinarian immediately for a proper diagnosis and treatment plan.
Here are some possible causes of these symptoms:
* Hypothermia: This is a drop in body temperature, often caused by exposure to cold weather or water.
* Infection: A variety of infections can cause fever and lethargy.
* Pain: Shivering can be a sign of pain, especially in the muscles or joints.
* Underlying medical condition: Other health problems, such as kidney disease, liver disease, or thyroid issues, can cause these symptoms.
Please don't wait to seek veterinary care. Your dog's health is important.