A stranger or friend says, "You must have really loved him," and the speaker's whole inner world shifts in a few short lines. The first thing she does is look down, a small gesture that suggests she can't meet the other person's eyes. The comment seems to bring her face to face with something she's been avoiding: what she has given up, and what she now has to do. The poem never says what that task is, and the speaker doesn't spell it out, which makes the moment feel heavy and private.
Her reply is just two words: "I did." The past tense matters here. She doesn't say she loves him, only that she did, so the answer quietly admits that the love is over, or must be. The mood is sad but calm, even restrained. Nothing is shouted or explained. The short, plain lines mimic someone holding herself together, saying only as much as she can manage. What's left unsaid carries the real feeling.