Why do I keep having dreams that my boyfriend is cheating on me?

Three-ish weeks ago, I was paranoid about him and his ex hooking up (we’re in a long distance relationship), but since then I’ve rationalized it and stopped worrying about it. While awake, I really haven’t given it much thought at all, but I keep having vivid dreams about them being together. Why do I keep having these and is there any way to get them to stop?