The reason for this is really character development things, but you know. I'd like to have a...reason I can give the readers, since it is something rather odd.
Right now I'm thinking along the lines of head trauma as a young child. Is it possible that some sort of trauma (ie, getting hit rather hard with a bat, for example) would damage the emotional center of the brain, making it nigh impossible for her to show any emotions that she does happen to feel? (I know somebody once who got hit in the back of the head, toward the base of his skull, and it totally decimated his sense of taste and smell. That's hardly the same, but it's a similar vein, so I'm thinking it's plausible.)
If not, are there any injuries or disorders you can think of that would explain this phenomenon?