A couple of possible reasons come to me:

1.) Maybe someone she cares about came back into her life, right about the same time as your date with her??

2.) Maybe you misinterpreted her demeanor on your date?? Perhaps the great chemistry was more one-sided than you interpreted and she was just "'going with the flow"?

3.) Maybe she just needs a little bit of time to compare you to others on her list??

Those are the only thoughts that I had about the situation. Best wishes to you!