The girl from your childhood may have shown up because someone in your current life reminded you of her. Or the dream may have to do with something in your past, approximately the same time you knew her.

When she throws the rat at you, what comes to my mind is the expression to be "ratted on" - meaning, that someone has told something about you that you don't want known. The fact that the rat is dead may mean that it's not a current issue - not quite as harmful, perhaps, as a live rat. Whatever it is that you don't want known, whatever transgression you would rather keep hidden, it's in your past, not current.

You knew as she approached that she was up to go good. So this is probably something that you saw coming a while ago - maybe you trusted someone against your better judgement, and could tell that it was only a matter of time before your secret was out.

I hope this helps you some.