Hello Marlene, I'm Dr. Deb.
I recently came online and see that your question about Jasmin hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
There could be several explanations for behaving as you describe. Unfortunately there is not just one reason that would explain her change in behavior. She could have hissed just because she wasn't feeling well and didn't want to be bothered but there are other reasons which I'll mention below.
1. She could be running a fever. If you have a rectal thermometer, you could take the temperature although this is often very hard to do in a cat. Normal is between 100 and 102.5. Of course, then we'd have to wonder why the temperature is elevated. Explanations for this could be: a bacterial infection such as an abscess (which is why she was hissing when you moved her) or possibly a viral infection.
There is a condition in cats that is called Fever of Unknown Origin--this is exactly what it sounds like which is that we can't find an explanation for the elevated temperature. Treatment is primarily supportive which consists of fluids and drugs to reduce the fever. Most cats recover very quickly once treatment in initiated.
2. Pancreatitis can happen very suddenly; these cats just stop eating and can become lethargic. We don't have a good explanation for why this happens, either. We do have a test for this (spec fPL) although I've come to doubt its reliability in some cases; treatment consists of fluids and pain medication (we believe the pain is why they stop eating).
3. Other systemic diseases such as kidney disease, liver disease, anemia, problems with the heart, etc. would all have to be included on the list, given her age. In most cases, cats are able to mask disease until it catches up with them.
Blood work will rule in or out many of these conditions if she has to be seen and a physical exam doesn't detect any abnormalities.
4.Cats who are constipated can behave this way but I'd expect that she might be frequenting the litterbox and straining to produce a stool....although I have seen a few who didn't.
5. Some cats will just have "off" days which typically last 2-3 days at most.
6. She might have some arthritis or spinal pain even though it seems like her abdomen is the source or her discomfort.
I'd dearly love to be able to suggest some over the counter option for you to consider but since cats are so sensitive to many drugs, it would be best to have a better idea of her underlying problem before making such a suggestion.
Maintaining hydration with water or Pedialyte is important, especially if there’s a fever. You could also offer her low sodium chicken broth or tuna or clam juice just to get fluids into her. You could also try dribbling some liquids into the side of her mouth every several hours.
I hope this helps although, again, my profuse apologies for the delayed reply. Deb