Once your dog is checked out by the vet, including x-rays, blood and urine tests, you can work on the weight issue. If she is developing a chronic illness you might need to work with a special diet. Older dogs do lose muscle mass and need higher levels of protein than adult dogs. It would be super easy to add some quality protein like an egg or sardines or hamburger patty to her regular food on a daily basis or perhaps switch to a higher protein senior kibble. More exercise is needed to go along with the added protein.