Shaking is often a response to pain so you need to see the vet with her to find out what's going on.
This could be as simple as a parasite problem (the number one cause of lethargy) or pain caused by overly full anal glands but you won't know until you see the vet.
I suggest that you schedule the vet visit (avg cost between $35 and $50) and then go from there.
I hope this helps!
Let me know if you still have questions.