D.E.
I'd tell her the truth. Chances are, on some level, she already knows something is really wrong. She probably can pick up on your anxiety and discomfort about all this and without information to go along with it, it is very disorienting for her.
It sounds like she's having a very normal reaction to grief and fear. Hold her, empathize ("Yeah, me too. I know how hard it is to not know, and to feel so scared...You aren't alone, I'm right here, and I'm so glad you can share how you feel.") I think she'll eat when she gets hungry, and it's normal to have no appetite when grieving. Make sure she drinks fluids though.
I think a mistake we parents often make is to try and minimize or prevent sadness and despair. But these are normal feelings to experience, and it would be a disservice to her if inadvertently she got the message that it wasn't okay to feel what she feels, or that her pet wasn't that important, or shouldn't be.