In chapter 18, when Darcy and Lizzy are dancing at the ball, Lizzy says, "...my head is always full of something else."

And Darcy replies, "The present always occupies you in such scenes-does it?" said he, with a look of doubt.
Lizzy then replies with, "Yes, always," she replied, without knowing what she said.

What does JA mean by all this? Why should Darcy think it bad that Lizzy thinks always of the present? And why is it implied that Lizzy did not mean to say what she did?
I think that from what Lizzy has said she should be thinking of Darcy at that moment, because she is dancing with him, but I'm not sure. What does anyone else think?