Hit a nerve

A: You seemed to have hit a nerve when you brought up Mary’s former boss.

B: Why?

A: She went silent for a moment. In fact, they had a three-year relationship before he fell in love with a blonde.

Note: This idiom means “to evoke a strong emotional reaction, such as anger, sadness, or disgust, upon being encountered, heard, read, etc.” You often “hit/touch someone’s nerve” by saying something that upsets them because they are very sensitive about that subject. For example: You hit a raw nerve when you talked to the manager about the need for better communications within the company.