Could it be a stomach ulcer? Or possibly even cancer? I'm not a vet, but I know these two diseases can cause weight loss. Hard to say without having more info. How old is she? It does sound strange that she keeps losing weight - definitely have it checked out by the vet.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare