The fact that he didn't even flinch was rather upsetting, but not as upsetting as the fact that her hand still stung painfully from hitting him. "Do not talk to me like I am some child who is overreacting," she told him, even if he possibly hadn't been treating her that way. It could have simply been her anger getting in the way of her judgment.
"I will never love you again," she hissed, once again slapping him, in hopes that this time he would react. It was rather upsetting that he hadn't the first time.