Most Helpful Girl
Well, you don't have any "right" to react to it at all because you aren't in a relationship. He's free to do as he pleases, and so are you. This does sound like a bad sign though, especially if he did spend the night with her. The real questions are these: Do you trust what he's said about not having a romantic interest in her? If you trust him and have "a thing," then why not make this a relationship?