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Dr. Paul Brazzell.
That is an old cat :).
Could be a lot of things: Behavioural, diabetes, hyperthyroidism, liver or kidney disease, low potassium to name a few.
I would strongly recommend a physical examination and blood and urine tests. That is the only way you are going to get to the bottom of this.
Please feel free to review the findings with me when you have the results back.