Well, if the bloodwork doesn't show anything being wrong, did they examine her teeth? Lots of things can contribute to a lack of appetite. And if her mouth hurts, of course she wouldn't want to eat. Any other changes in behavior?

If the bloodwork says nothing's wrong, maybe ask about some appetite stimulant, or I'd get a second opinion from another vet if you can. That's a lot of weight to lose so quickly ... How much did she weigh before? And now?