You upset her and then try to talk/play with her. Once the relationship increase enough there will be thunderstorm event. Choose to be kind to her will unlock a more friendly end. And at the last second of friendly end, you can ask her to be with you and that's when the real lovey-game begin.There'll be another choice event before day 19. Choose wisely, for if you do, you'll unlock the choice to have her stay.
Also, you sure you didn't lewd her or her things in any way? Check the stat page, if she sees you as an ATM, you fucked up somehow.