Hello @januaryjane
As you know, those of us on Mayo Connect are not medical professionals so we cannot diagnose problems like yours. We can, however, share from our own experiences. I have a form of Parkinson's. I do not have the typical tremors but I do understand the feeling of internal tremors. I have met many people in Parkinson's support groups who have also talked about the internal tremors.
Seeing a rheumatologist is a good idea, and you might also ask for a referral to a movement disorder specialist. This is a group of neurologists who treat specific movement disorders such as Parkinson's and other disorders. Perhaps they could help you with a diagnosis as well.

Will you keep posting and let me know how you are doing?