I was hoping to get a response back before answering but I don't want you to go without an answer either. This could be related to pain or even be seizure activing.
Shaking or trembling in toy breeds can be due to hypoglycemia, liver shunt, or a neurologic response such as a seizure. Trembling can also be a sign of pain, stress and cold.
Hypoglycemia is low blood sugar and frequently causes trembling in small breeds. Feeding your dog smaller meals more frequently can help. If you suspect your dog is having low blood sugar you can put a drop of pancake syrup on your dog's tongue which should raise the level. If a small dog stops eating for a day or so, it can bring on low blood sugar.
A liver shunt is usually a genetic condition. It is a condition where instead of the blood going through the liver and being cleansed, part of the blood is diverted around the liver resulting in a toxic buildup in the blood. If the liver is not functioning correctly you might see similar symptoms as well. You can read about these here:
and liver dysfunction here:
Seizures can manifest as trembling and a dog may not lose conscienceness. Focal seizures can manifest as repetitive behavior or just staring out into space for a minute or so. You can read more about seizures here: http://www.marvistavet.com/html/body_seizure_disorder.html
Pain can also lead to pacing, panting and trembling as well. Given her age, I have to recommend you have her seen by your vet and have a senior checkup done.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer so I am compensated for my time.
If you have questions in the future that you wish me to answer, you may click here and bookmark the page or make it a favorite. It is best to put my name "JANE" in the question as well.
Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.