She didn't change 'her mind', her emotions changed. Which is to be expected when she's in the emotional shitstorm following a breakup.
What I don't entirely understand is why you didn't shift the date at your place is everything was going so amazingly well.
The Joker 1308 wrote:
Am I an idiot here? Am I missing out something?
You're missing out on the fact that you're a rebound.
She newly single and likely wants to have some fun. What she doesn't want is to jump head first into another commitment. Stop thinking "long term" after one date. That kind of implicit pressure is what will scare her away.
If you want something to come out of this be light and offer her the excitement she wants without the unnecessary string. And don't take it too seriously.