Yeah I think Kodi totally got it right. Everyone knows their partner. If he think she's interested in someone else, she probably is. If she thinks he'd bang her best friend the minute she walked out the door after a fight, I bet he would.
James, if you really think she's judging you unfairly, ask yourself if there's a possibility you're still creating options for yourself that you might exercise when this relationship ends (or maybe even as a way to end the relationship).