My Partner's Secret

I recently found out my partner of two years has been seeing someone else. We're in an open relationship but I had a rule about meeting people through apps or outside our social circle. She told me it was just casual, but the way she described him made me feel like there was more to it than that. I'm struggling with how much this affects me. It's supposed to be easy for both of us to have other connections, but finding out about him makes my stomach churn every time we're together now.