having not been there my only guess is that she hit her high above the spine. If you look at the anatomy of a deer the spine is lower in the body then you might realize. If she hit her high it would seem that the shock the slug hitting her temporarily stunned her which would explain her flipping onto her back and appearing to be hit very hard. Once the initial shock wore off she would be able to recover and run away. Since the wound is really nothing more then a flesh wound it would make sense there would be some fairly good blood where she went down and thrashed about. it would also explain the lack of blood when you attempted to trail her. if you put a slug through anything vital she would be down in very short order, if you hit guts she probably wouldn't have flopped and you def would have found guts. My guess is that she is probably still out there walking around, likely pretty sore but other then that no worse for wear...they are remarkably tough animals. Chalk it up as a learning experience, you can teach your daughter that when in doubt shot again and to work that action immediately after the first shot so she is ready. Also teach her that when you work the action on an 870 to slam it back and slam it forward instead of short stroking it and hanging up the empty (although i understand your reasoning in trying to sneak the shell out quietly). tell her to keep her chin up, maybe you guys will get another shot at her later this season