I was also going to suggest a hidden dental problem. At 15, her teeth could definitely be getting fragile and prone to fracturing. Happened to mine last year (at age 14); he had refused (or been unable) to eat for a day when I took him to a vet who quickly glanced in his mouth when I suggested it and said "looks fine." After another night of no eating or drinking (I was using a dropper to get fluid into him), I got a second opinion; this vet looked more carefully and found not only a broken molar, but an abscess starting. Had to have it surgically removed, but he made a full recovery. He now eats only wet food. I would pursue that possibility until it was definitively ruled out. Good luck!