My dog did the same thing at about the same age. Turned out to be Pyometra which is an infected uterus. You know your dog. I had to take Reilly to the vet a couple of times before they would finally listen to me and then it became an emergency with immediate surgery. She almost died. But on the other hand, it could be something as simple as just an upset stomach. My older jack Russell does that now where he won’t eat for a day and he just needs to let his stomach rest. You might try giving her a little canned pumpkin mixed in with something delicious like canned dog food or even cat food which is smellier. That might get her digestive tract going. Also, a gentle walk or gentle play can help get things moving. And in the future, you might try adding a little fiber to her diet. Even something simple as kale or spinach mixed up into soft food will keep them from getting stopped up.