Take her to the vet to be examined and see what he says. You don't have to have her put down but if she is in pain it would be the kind thing to do as you wouldn't want her to suffer. Othewise if she is just winding down to old age, keep her comfortable and offer her food in case she wants some. She has spent her whole life with you and upsetting though it may be just continue to love her until the time comes for her to sleep peacefully. xx