Perhaps the slap had killed something inside her, and that something had made her so numb she did not even cry.
The hand that had once caressed her cheeks and wiped away her tears whenever she went to him after carrying the heavy burden of not being loved as equally as her sister—the hand that had held her in a gentlemanly way against his chest and whispered to her that everything would be fine once he married her and got her away from the Dawson household, even though she always told him then that it wasn't her family's fault—had hit her now.
When you thought you knew someone so well you could swear to anyone that the person was this certain kind, only for that person to be different, it made one realize you had loved an image you only wanted to believe in and not the real person. One could not suddenly change unless you had that personality in you all along.