Someone has just walked away, and the speaker is left watching them go. The poem opens with a physical feeling: an emptiness in the stomach. That's a good choice, because grief and loss often hit the body before the mind catches up. The speaker also seems certain that this isn't a short goodbye. They know the person will be gone "for a long time," and that knowledge hangs over everything.
The second half shifts from feeling to questioning. The speaker can think of nothing else except "Why did you leave?" and "What did I do wrong?" These lines are plain and almost childlike, which makes them sting. Instead of anger, there is self-blame, as though the departure must be their fault. The mood is quiet, hollow, and unresolved. The poem gives no answer, so we're left in the same place as the speaker: stuck on the same questions, with nobody there to answer them.