Her posture intensifies a bit, her head being lowered a bit more, but she still glares back at him.

It's not information that she really wants to give him, if that means that he'll try to talk her into getting rid of the locket. And she can't answer part of it anyway. So her answer is incomplete. "It was in my things, I already told you that. I just don't know how it got there, it just was there one morning."