Is he cheating?

My bf 35/M and i 31/F have been dating for about 5 months. I was with him the other day and he pulled up his Facebook… and saw he was apart of a local dating group and this woman had posted a selfie. I said whoa what is that? He said oh i don’t look at it don’t worry. But i joined the group and it turns out he joined it 2 weeks ago, and I was with him that weekend. Please help. I’ve never felt this kind of pain before. He is a Christian man at a local church.