Welcome to JustAnswer! I am a licensed veterinarian and specialist and will do my best to assist you today!
Does this happen when she is sleeping or awake?
Starts when she is sleeping but continues when she is awake. Only happens as she trys to settle down to go back to sleep. She will eat and otherwise seems ok
Is she alert and responsive during this?
Ok - excellent.
This is then unlikely to be seizure behavior.
As seizures would also be described differently.
My vet felt the same
This is more likely related to causes such as anxiety, stress, pain.
It is not likely viral such as distemper, seizure behavior, or organ / metabolic disease.
She doesn't seem to be in pain and she is not a classic jack in that she is very quiet. She doesn't seem stressed or anxious.
It is not seen in many dogs, but some dogs just have idiopathic (unknown cause) tremors.
Otherwise - if routine testing has not found a cause - they are getting worse and affecting quality of life, I would try to video an episode and either show your veterinarian or ask your vet for a referral to a neurologist in your area.
You are very welcome!