The speaker admits to a small, quietly painful fact: since losing someone, they've kissed another guy. The poem doesn't brag about moving on or confess to guilt in any dramatic way. Instead, the kiss sets off a strange question. If the ex ever kissed them again, would the new person still be there, somehow, on their lips?
That final line is where the feeling sits. It's an odd, intimate image, and it suggests the speaker isn't over the relationship at all. They're still imagining a reunion, and they're worried about what has changed. There's a hint of guilt, too, as if the new kiss has left a mark that the old love could detect. At the same time, the question might carry a little defiance, a wish that the ex would feel what they've lost.
The mood is hushed and uncertain. The short lines and the lack of any answer leave the poem hanging, like a thought the speaker can't quite shake in the middle of the night.