I have to say that I am quite concerned about your wee one’s condition.
Progressive abdominal distension to the point of discomfort in what sounds to be a young cat is terribly abnormal. If the abdomen feels firm, then we'd have to consider issues like organ enlargement, tumors, and significant fluid distension in this belly.
Now the first two are self-explanatory, but in regards ***** ***** in the belly we can see a range of causes. One to consider is fluid build up due to a heavy burden being present a long period of time. When this arises, we can see belly distension because the worms steal blood proteins through the gut. When blood proteins get low this disturbs the oncotic pressure of the blood vessels and can cause water to leak from the vessels and pool in the abdomen leading to a big belly.
Other possible causes would be if she has a heart issue (at her age it would be likely something she was born with). The problem with heart defects is that the diseased heart won't pump as efficiently as it should, which leads to disturbances in blood pressure in the blood vessels. Too much pressure and the vessels leak, and serum accumulates in the body (often the abdomen). Often kitties will act totally normal with this but in later stages we can see them be less active, develop big bellies, and may show signs of struggling to breathe. For these cats, they can be treated with medication (diuretics, and drugs to make the heart beat more efficiently) or surgery (for certain types of defects only). But overall treatment success is very dependant on the particular defect and the current progression of the disease.
As well, while it isn't nice to think about with a young cat, we can also see tumours cause a similar issue. They are uncommon but we can see them and some will be linked to the FeLV virus. But if a tumour is present it can cause distension itself or can affect the blood supply to cause a similar pooling of fluid in the abdomen (often with cells of tumour in the fluid's suspension).
And the other cause of ascites that I am particularly worried about in her situation is Feline Infectious Peritonitis (FIP). This is caused by a virus that is known to also cause diarrhea signs in the cat. It causes the accumulation of fluid in the abdomen because the virus triggers an inappropriate immune response and together they erode blood vessels making them leaky. The cats often keep eating and drinking despite the progressive belly girth. You may notice that the gums will become pale and/or yellow. This change will likely be present at all mucous membranes (conjunctiva, roof of mouth, etc). In cases of FIP, palliation can be tried (with medication to block the virus and dampen an overzealous immune system) but in most cats this viral infection is fatal (it is truly a horrible virus that targets young cats).
The key thing here in a kitty with severe belly distension of this timeframe is to have your vet evaluate her (it isn't an emergency but I would say that it would be worth getting her checked when your vet as soon as possible). They will listen to her heart and palpate the abdomen. If they agree that her belly feels abnormally distended then they will likely put the ultrasound probe on her belly to confirm this. If large volumes of fluid is seen, then it would be prudent for them to take a sample of the fluid. Identification of the type of fluid will determine what you are dealing with. Depending on their findings, they may need to scan her heart (if its heart troubles), or check bloods (if its FIP), or send the fluid to the lab (if there is a tumour cell they can't identify).
Overall, even though she is otherwise only showing mild signs, this belly distension is very abnormal and needs to be evaluated by your vet. They will be able to rule out what it isn't and advise you of what is causing her distension. Once they have an idea of which of these is to blame, the vet will be able to help you address this and treat her appropriately.
I hope this information is helpful.
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All the best,
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