I would call the shelter again and if they don't call back or there is no one there to give you some advise then call another. There has to be someone that can help. And yes it is possible that she ate the still born kitten. It sometimes happens. When a kitten is live born the clean them and eat the placenta so if the baby was still born it wouldn't be that strange for the mamma to do that as she would know there was something wrong. But you might want to have a look around where she is as she may have hidden it too. And also look and see if maybe there are other kittens. Is there anyway for you to touch her belly? At least if you could do that you could feel if there are more babies inside. I know she will probably bite and scratch but we are talking about possibly saving her life especially if there are still babies inside.