a raw nerve

An intensely negative emotional response to something. Used especially in the phrase "hit/touch a raw nerve." I could tell he was touching a raw nerve when he brought up Jane's former employer, because Jane started staring at him with white-hot fury in her eyes. She has a penchant for hitting raw nerves on her show, so it doesn't surprise me that she's gotten so much hate mail. I'm sorry, I didn't mean to touch a raw nerve there—I didn't know that you and Lydia used to go out.