Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that your older girl isn't feeling herself with a recent history of loose stools that seemed to improve with not feeding canned food, and now eating poorly and thus producing less stool.
A decrease in appetite can be related to eating something she should not have, too many treats or table food, eating foreign material (which could cause a full or partial gastrointestinal obstruction), chronic pancreatitis, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, heartworm disease, hyperthyroidism (a tumor of the thyroid gland) internal organ failure, pancreatitis, or even infiltrative cancers such as lymphoma.
I do think that this is concerning. In an older cat we do need to worry about more serious diseases that interfere with her appetite, such as organ failure or cancer.
Because she hasn't eaten normally in several days and she is producing less stools I am very concerned about her. If she isn't eating normally her liver will start to break down fats for energy to live, become overwhelmed and unable to preform its normal functions, and she may develop a type of liver disease called hepatic lipidosis.
Ideally she would see a veterinarian today if this has been going on for more than 72 hours. Simple stomach upset should pass within 24 to 48 hours. They could examine her, run blood tests and possibly check radiographs and/or an ultrasound to evaluate her and know best how to treat her.
In the meantime they can administer injectable anti-nausea drugs and fluids to rehydrate her.
If you cannot have her seen for whatever reason there are some things you can try at home.
At home to try and settle her stomach and get her to eat better you can give either:
1) Pepcid ac (famotidine) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at a dose of ¼ of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours
These are acid reducers and may help her feel less nauseous so that she will eat better. They are quite safe and can be used long term if necessary.
You can use a medicine syringe to try and force water into her orally if she doesn't seem to be drinking well.
A couple of hours after giving the acid reducer I recommend offering meat baby foods or a bland diet of 2/3 boiled minced white skinless chicken and 1/3 boiled white rice mixed with some low salt chicken broth to make it easy to lap up and swallow.
If she continues to not eat well she should see her veterinarian for an examination, diagnostics and intravenous fluids and supportive care.
Please let me know if you have any further questions.