Its been implied in the series before, that Fluttershy has some serious repressed anger issues. Which may lead to a possible split-personality of sorts. But how do you think Fluttershy vents her anger in order to maintain her kind reputation? I personaly would like to think she keeps her true emotions recorded in a secret journal nopony knows about. Where she records her thoughts about her friends, family, and the world around her. Either that or she argues with herself in the mirror when nopony's looking. This could explain how she is usually able to control her inner demon.