What does "your loss" mean? I HATE when girls use it. To me it says, I kinda wanted you to come to this, but since you have other plans, I'll save face by being a slight dick. Perspective.
Dear Anakin Skywalker,
You're not far off with that one. "Your loss" is a way of turning the disappointment off yourself and onto the other person. It always follows some kind of rejection--whether it's breaking someone's heart or a booty call being denied. I've experienced both sides of the "your loss"--I've heard it and yes, I've said it. And yes, it's kind of a dick move. But it's a defensive dick move. It's a way of saying "I could care less about this rejection--I pretty much won't remember it in five minutes--but man are YOU going to regret it." On the surface, it's "I don't give a fuck"...but underneath that it says "I CARE SO MUCH."
Remind yourself that she's probably really into you and let that boost your ego for a bit, but let it slide so she can save face. She really does give a fuck that you said no.