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Hello. Thanks for the question.
I would probably get her seen if it's been several days that she is lethargic and not eating well. Given her age, I'd run some bloodwork on her. That will help rule out kidney disease, liver disease, diabetes, infectious causes and auto-immune causes. If the bloodwork is normal, then I'd treat symptomatically with an appetite stimulant (Mirtazapine), an anti-nausea med (Cerenia) and potentially some GI antibiotics (Metronidazole).
If the bloodwork is abnormal, then we start working up further and consider hospitalizing her. Her signs are pretty vague, so there are a lot of possible causes and treatment protocols. I'd want to start from square one and then go from there.
I hope this helps, let me know if you have any other questions.