would this be considered cheating?

My boyfriend has seemed very happy and nothing seemed like he was unhappy or bored with me. Someone sent my friend a video of him in class and there's a girl flirting with him and he reaches down and slaps her thigh. he immediately takes his hand off and looks away with a straight face and clenches a fist like he knew it was wrong to do. I asked him about it and he said that he's really happy with me and he doesn't know why he did it. He said that he loves me and doesn't have any feelings for her at all. Whenever he is sad or mad he'll clench his fists and just shake. While I was talking to him about it he did that because he thought I was going to brake up with him. I told him if he ever does again that we're done and he promised to never do it again. I gave him a second chance but I honestly can't just let it go. I feel like he's gonna do it again because he's broken my trust and it just really bothers me and I don't know what to do. Am I over reacting? What should I do? What do you guys think? He also never told the girl me and him are dating.