You didn't say what breed she is. If the vet has checked her and has no easy remedy, it may be her time even if she is a smaller breed you could have hoped for a longer life. You may want to read the sticky starting at
http://www.askmehelpdesk.com/dogs/in...tml#post254171 You may want tempt her with rich food to give her a little more time if you don't think she is suffering. Certainly it is no time to worry about the long term consequences of an unbalanced diet. My friends went through this with their 15 year old Lab Maggie last fall. Maggie was allowed to decline for a month or 2 giving us all a chance to accept her time had come. That Maggie was deeply loved by 3 families, her host family, my family that raised Aster from her first litter, and the lady that was Aster's partner the years she worked as a dog guide. After losing Maggie, it would have been very hard for all of us if I hadn't spent $1200 this year having a tumor removed. Although nearly 14 now, and slowed by arthritis, Aster is still enjoying retiring to her puppy home.
As I start out in the sticky, the important thing is to know your dog.