She could be stressed... what are your temps and how are you measuring them? She might not feel secure in her enclosure and be killing the mice out of fear and not hunger. Try loosely crumpling many balls of newspaper and putting them in her cage. This way she will be able to cruise around and still have some cover to feel safe. Make sure her temps are good(92-94 hot side & 80-82 cool side) and that she feels safe in her home and she will eat for you. Good luck.