Let me guess: female? She's not interested if she does that. She probably already knew she didn't want to hang out, but gave you a vague answer as to when she was available because she couldn't think of a quick enough reason not to be. She didn't want to seem rude, even if it meant letting you waste your time so that you wouldn't be wasting hers.
Don't ask for an explanation from her and don't accept it if she gives it. You just castrate yourself.