The most common causes of unexplained weight loss while still eating and drinking are the following: Diabetes, Hyperthyroid, gastrointestinal ( GI) cancer, and inflammatory bowel disease (IBD).
If she has already tested negative for diabetes and hyperthyroid, that leaves GI cancer and IBD.
IBD sometimes improves with the use of oral or injectable steroids, a novel protein hypoallergenic diet, symptomatic medication and herbal formulas. GI cancers can be benign or malignant and ultrasound may help to diagnose a GI cancer or other abnormality.
Thanks so much -- you gave more information than the vet!