Yep I agree with the other two, I would think pain first. My dog is 13 and has arthritis. I worked as a vet nurse for 13 years, so I kinda know the signs, but her first things were behaviour issues. Shaking in dogs is mainly a sign of pain over even chills. I personally would take her to a vet and get her checked over for arthritis and senility. Our old girl is now nearly completely deaf, and blind and has arthritis. Its sad seeing our furbabies grow old