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Hello. I'm sorry to hear about your cat.
A distended abdomen in an older cat like this is often due to fluid accumulation. That can be secondary to cancer in the abdomen (Lymphoma). It could also be secondary to low protein and leaky vessels. This can be a result of protein loss in the urine from kidney disease. Lastly, it could be due to right sided heart failure.
I would get her seen in the morning and get the abdomen tapped. The type of fluid will help determine the underlying cause and the next steps regarding diagnostics and treatment.