Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that your girl isn't feeling well and is lethargic, vomiting any food she is eating. It makes sense that her stools are getting much smaller because she isn't eating enough to make stools, and of course if her calorie intake has fallen drastically she will lose weight and feel weak.Vomiting 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, esophageal reflux, a dietary allergy or sensitivity, toxin exposure, a viral or bacterial infection, inflammatory bowel disease, heartworm
(a tumor of the thyroid gland) internal organ failure, or even infiltrative cancers such as lymphoma.Since she isn't able to keep any food down she will quickly start to break down muscle and body fat for energy to live, and as her liver breaks down fats to live she may develop a type of liver disease called hepatic lipidosis. This can be fatal if not treated aggressively.I know that she has seen her veterinarian but because she didn't respond as usual she should be rechecked.Was any bloodwork checked? 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 a cat that was sickly as a youngster, is small in size and has had chronic respiratory signs I would be very suspicious of chronic, immunosuppressive viruses like Feline Leukemia, Feline Immunodeficiency virus, or Feline Infectious Peritonitis. These will suppress the immune system, can cause infiltrative cancer or granulomas to form leading to a possible intestinal obstruction. They can pass from mom to kitten, or be picked up from other cats as a youngster, smolder for years and then lead to serious illness later.In the meantime to support her your veterinarian can administer injectable anti-nausea drugs and fluids to rehydrate her.If that absolutely isn't possible today there are some things you can try at home.At home to try and settle her stomach you can give either:1) Pepcid ac (famotidine) at a dose of 1/4 of a 10mg tablet per 5 to 10 pounds of body weight every 12 hoursOR2) Prilosec (omeprazole) at a dose of 1/4 of a 10mg tablet per 5 to 10 pounds of body weight every 24 hoursThese are acid reducers and may help her feel less nauseous so that she will eat and hopefully not vomit. They are quite safe and can be used for several days if necessary.You can use a medicine syringe to try and force broth or water mixed with meat baby food or blenderized canned cat food into her orally a couple hours after giving her one of the acid reducers.If that goes well 12 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 vomiting she should see her veterinarian for another examination, diagnostics and intravenous fluids and supportive care.Please let me know if you have any further questions.