Hi, and thanks for your question.
This is a quite generic but concerning presentation. Differentials are many, from gastrointestinal diseases (infection/inflammations often dietary related) to any disease that can spread systemically (causing lethargy for example) like infection, and anything that can cause pain/discomfort.
The approach must be logic, starting from offering different kind of food. A check up is needed if inappetence lasts for more than 24hours.
A more aggressive treatment is certainly needed after diagnosis has been correctly established.
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