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Hello and thank you for using this site to help you. Trembling and panting can be a sign of pain or fear. Even though she does not seem like she is in pain, she may be as dogs are stoic in the sense they don't always cry out.
Given her age, pressure on the brain can cause seizure activity and with focal or aura seizures you would see a change in behavior and many dogs with these become clingy to the owner. Try to notice other times of the day for odd behaviors.
Older dogs can also go through night terrors, or old dog syndrome which is similar to human dementia.
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