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If you have not seen the vet recently for a fecal exam, this would be a good time to do so. The most common cause of lethargy and inappetance in dogs is an undiagnosed parasite, giardia or coccidia.
I would see the vet tomorrow for a fecal exam to rule this out and then go from there.
Let me know if you have more questions.
It's pretty hard to say what's going on when the only symptom is lethargy.
If she was mine, I would switch her over to a bland diet of only boiled chicken and white rice. This will help if it's a stomach problem. If not, it will still get some food into her.
Then I would re-evaluate the symptoms tomorrow and go from there.
You are very welcome!
Just plain boiled chicken and white rice. No spices, no onions no garlic. This will soothe the stomach and get some nutrition into her.