This is Dr. Christian. Let's see if I can help you. Is she vomiting or having diarrhea? Does she drink a lot of water or urinate more often?
There are several conditions we have to be concerned about in this case.
1. Hyperthyroidism- This is caused by a tumor in the thyroid gland that causes overproduction of thyroid hormone. It causes a cat to have a very high metabolism. The tumor does not spread to other parts of the body. Symptoms are eating with weight loss, vomiting, increased thirst and urination, diarrhea, fast heart rate and heart diseases. It is a very treatable condition with medication or radiation treatment. Diagnosis is by bloodwork.
2. Diabetes- Diabetes is a lack of control of blood sugar. The blood sugar becomes very high. The symptoms are weight loss, increased urination and thirst, more prone to infections, occasional vomiting and eventually loss of appetite and weakness. Treatment is usually a diet change and insulin injections. It is diagnosed with bloodwork and sometimes a urinalysis.
3. Kidney Disease- Kidney failure is very common in cats and is a condition that is not curable but certainly can be treated to improve a cat's quality of life. Symptoms include weight loss, vomiting, increased urination and thirst, and lack of appetite. Diagnosis requires bloodwork. Treatment usually includes diet changes, fluids and sometimes medications to improve anemia or lower phosphorous levels.
4. Cancer- Cancer is a common condition as well and can cause a varied list of symptoms including weight loss, diarrhea, vomiting, lethargy and lack of appetite. Some cancers are more treatable than others depending on the type.
5. Inflammatory Bowel Disease- Inflammation of the wall of the intestine will cause a decreased capacity to digest food leading to malnourishment. Symptoms include diarrhea, weight loss and vomiting. This is sometimes caused by a food allergy. Diagnosis is usually made with an intestinal biopsy but many times treatment is prescribed to monitor for improvement. Treatment usually involves steroids, diet changes and sometimes antibiotics.
6. Liver Disease- Liver disease can be caused by cancer, infection or inflammation (hepatitis). The symptoms include weight loss, lethargy, vomiting, diarrhea and sometimes a yellow appearance to the skin called icterus or jaundice. Diagnosis usually starts with bloodwork but final diagnosis of the cause usually requires ultrasound and a biopsy. Treatment includes diet changes, possibly antibiotics, and liver medications to help improve its health and function. Treatment is also specific to the cause of the liver dysfunction.
7. Feline leukemia and feline aids viruses- These viruses are spread from other cats through bodily fluids. Unfortunately they are not curable and ultimately deadly. Common routes of transmission include bite wounds and from mother to kitten. Simple blood tests can diagnose these diseases and vaccines are available. Treatment is usually supportive such as fluids and medications to help appetite.
If your cat has been tested for leukemia and aids and is not any outdoor cat than it is less likely these are the cause of the problem. While she is not showing some of the symptoms for these conditions sometimes the only symptom in the beginning would be weight loss. I would recommend taking her to your vet. Some simple bloodwork can be very effective in determining whether any of these conditions are causing the problem. If you have more questions or concerns please let me know.
I'm sorry. I misunderstood and didn't realize this had come on so suddenly for her. Certainly pain would cause some of the symptoms you are seeing. Abdominal pain and arthritis are some possibilities for that. However, some of these conditions I've described could also cause weakness and neurological conditions and loss of muscle mass. If she is that weak then I would take her to a vet tonight if possible. It does concern me if she is that weak and it came on that sudden as you've described. Fluids and pain meds may be necessary to get her feeling better temporarily while you figure out the root of the problem.