Is she actually eating (do you see her actually chew and swallow)? Is she keeping it down, or having diarrhea? Has her food been changed or modified lately (new brand, or "improved" flavor, etc.) Also, has *anything changed* in her home recently - from new air freshener to a person?
Have you tried feeding her just meat? (Chicken, chicken organs, sardines, etc.)
Unexplained or rapid weightloss is Not a good sign. I'd take her to a vet asap for blood and other testing and then look to fixing any diet or environmental issues.