The reason for weight loss is negative caloric balance. The simple reason is that more calories are being burnt than the calories being taken in. There can be three conditions in weight loss.
1-Appetite Normal,
2-Increases
3-Decreased

The causes are
Metabolic
Dietary
Disorder because of poor absorption of food etc

For detailed information, visit the following link.
Weight Loss in Cats

 I believe if you take your cat to a Vet they will find a kidney dysfunction or failure. The same happened to my cat, although my cat is 27 years old (the oldest recorded cat in America by the way). Missy had become skin and bones but all of a sudden was driven to eat and drink lots of water. The Vet discovered kidney failure and suggested that we give her subcutaneous water injections under the skin. I told them that there was no way I was going to torture my cat in this way (the subcut injections take up to 5 min a day) because obviously at 27 years old she has led a long life and was entitled to die a normal death from kidney failure.

That was a year ago and Missy is still eating strong (I feed her smaller amounts of cat food, feed her about 6 times a day, and mix a couple of tablespoons of water in her food), and she drinks lots of water. I now buy various different brands of cat food as she is elderly and has lost some tastebuds. She hasn't put much of any weight on but her coat looks healthier and she is playing with her toy mouse again and walks the perimeter of the yard.

 She could have parasites in her gut - tapeworm will absorb all the nutrients from the food, making the cat very hungry but never benefiting from the food she eats. You should take her to a vet - if you can't afford the bills, look up animal rescue shelters on the internet, there's bound to be some in your area, and ask them if they can help you find cheap medical care for your cat. In the UK we have some big charitable organisations, for instance the Blue Cross, or the People's Dispensary for Animals, who will only ask for a donation, or charge a small fee.

 You might want your cat checked for worms as these pesky little things can cause your cat to lose weight and eat just as much or even more.

 Diabetes is another possible diagnosis, again you need to seek veterinary attention for her.

 She could have hyperthyroidism - It's optimal to have a minimum data base done (ie CBC, chemistry panel, T4, and a urinalysis)... It's always best to start with a full physical exam though first...

 If I were you I would get your cat look by a vet, there could be a number of reasons why your cat had lost weight.

 Hi there. When I read this, I thought I was reading about my own cat. She's about 13, has lost weight but is eating a lot. I will be taking her to the vet tomorrow ~ I will let you know what was diagnosed. However, you should take your cat to the vet asap as well. Good luck.