First of all I would remove the flea collar. They aren't effective at preventing fleas and while I don't think it has caused this problem it may not be helping the situation. Medically there are several things that will cause weight loss in cats. Here's some possibilties:
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. Usually the urine is very diluted so this may not be as likely.
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. This one also usually has diluted urine so it less likely.
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. To me this is a big possibility for your cat.
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. This is possible since she may have been exposed to other cats but pretty unlikely when she is indoors almost all the time. It would have to be pretty close contact for this to be spread.
There is not much for any of these conditions that can be done at home. Continue to try to give her as much fluids as possible. Put extra water on her food and feed her water with a syringe if necessary. I'm concerned that she will continue to go down hill without continued veterinary care however. If at all possible it would be best to take her to a vet. Your vet would probably start with basic bloodwork and go on from there. If you have more questions please feel free to reply back. I will offline for then next 30-60 minutes but will soon be back to answer anything else you need.