How does Eliza wound Higgins “to the heart”? (Act 4)

Eliza asks whether her clothes belong to her or Pickering, since he is the one who bought them. Higgins replies that of course they belong to her. When she protests that she did not want to be accused of stealing them, he is hurt. Eliza also gives Higgins back a ring that he bought her, a piece of jewerly that was not borrowed. He angrily throws it into the fireplace and says that she has "wounded him to the heart."