I think the song means that a girl has really fallen badly for a boy, but he either a.) is just faking along, or b.) doesn't know her at all. A. Is when he's just thinking that this isn't so serious, and B. Is when a girl notices him, and can't get her mind off of him. However, he doesn't know her.
She's just trying to get over him and not think about him at all, but it's not really working, because she's fallen for him bad, but he doesn't, and that makes it hurt even more.