Lost, I agree with UKVA - you don't need to get into specifics of why you can't meet up. Being vague creates mystery which will naturally cause her attention to perk up. Let her wonder why you can't meet up. If she throws a time out in the future, in the alternative of saying you're unavailable, you can just say, "Sorry, that doesn't work for me."
By giving her an alternate time to meet up, you gave her an opportunity to shut you down.
You are right to want to let her do a little of the legwork in terms of meeting up with you. But be careful that you aren't getting into the trap of tallying "wins" and "losses" for each of you - that can be very destructive to your DB mindset and in any future R with her.
