How did Caroline Bingley force Darcy into saying something that caused her pain?

Caroline purposely made disparaging remarks about Elizabeth and reminding Darcy of his own. This did not work to her benefit. ]

‘Yes,’ replied Darcy, who could contain himself no longer, ‘but THAT was only when I first saw her, for it is many months since I have considered her as one of the handsomest women of my acquaintance.’

He then went away, and Miss Bingley was left to all the satisfaction of having forced him to say what gave no one any pain but herself.