At 13, she may have teeth problems or excessive plaque that make it uncomfortable to eat. Have the vet check her teeth. At age 13 (very old for a dog) they begin to have health problems. In our house the old dogs get what we call ODP - old dog privileges. Right now I have an old lab, Duffy. He is 13 and can barely walk, but loves going out with the other dogs, and is generally very happy and has no pain. We give him a special dog food, ID from the vet and he is on Deramaxx, a pill that costs $2.50 a pill...but it keeps him going, comfortable and he still enjoys life.
Dogs that life a long time deserve special treatment for their faithful service to us. Hang on and give her special treats, one day she will be gone.