As a girl (who dates girls) I think she didn't want to seem easy by sleeping with you on the first night, and then felt that she'd let herself down by doing so. She must like you to have slept with you, and she obviously wants something more if she's agreed to date you. I think she's testing to see if you only want sex now she's slept with you so quickly (we're told that it's all that men want, and if you put out quickly then that's all they'll ever think you're good for). Don't force the issue of a relationship, just wait and see what evolves. And - most importantly - don't expect sex off her just because she slept with you so soon, let her know that you are interested in more than her body, and that she can take her time with you. She will be impressed with your understanding, and (haha) more likely to put out again!