Typically mice and rats have a different smell, and any difference can affect ball pythons eating. They are horribly picky eaters. As far as her starting off on the wrong end, it happens, doesn't really mean anything is wrong with your snake. My anaconda did it a few times, and I kind of re-directed her to the head, though I wouldn't recommend doing that because you may get bit. I would attempt to get a rat fuzzy(if that's what shes been eating).