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Hello. I'm sorry to hear about your cat.
I would definitely tap the fluid and try to get an idea on what the cause is. If the fluid is low in protein, then it may be due to a heart condition, low blood protein levels, liver failure or intestinal malabsorption. If it's high in protein, then it's likely due to cancer, a ruptured organ in the abdomen or FIP (feline infectious peritonitis).
There is not really a "good diagnosis" to give her of something that is fixable or curable. Given her age, FIP is less likely. Cancer is probably the most likely given the weight loss, normal appetite and fluid build up. Also, given that she's an exotic breed there are higher incidences of cancer in these cats.
I would tap the fluid and then consider x-rays and ultrasound to try to get a diagnosis.
I hope this helps, let me know if you have any other questions.