The speaker is in a relationship that has quietly gone cold. A month ago things were warm, but now something has changed, and she senses it in small details. He no longer calls her "baby" or his "special lady," and when he says he loves her, it sounds shaky. She notices that he doesn't look at her as though he sees paradise anymore.
The poem starts with a question, "Could it be that someone's in your head?", as if she's still giving him the benefit of the doubt. But by the line "I know there's three of us," the doubt is gone. She understands that he's been messing around, and she says it plainly.
The mood is sad but steady. There's hurt here, yet not much shouting. The final line is the most striking: she asks him to hold her one last time and, in the same breath, tells him she's leaving. It shows how much she still cares, and how firmly she's made her decision.