I (18M) was told one version of an incident by my girlfriend (18F), then saw a video that changed how I see it

A while ago, my girlfriend told me that during a girls’ trip she was forced to kiss another guy. According to her, he held her so she couldn’t move away, and the kiss was not something she wanted. She was extremely scared to tell me because she thought I would break up with her. I believed her, supported her, and reassured her that it wasn’t her fault and I wouldn’t leave her over something like that.
Recently, I found out more about what happened that night. I’m not proud of how I found out. I went through her phone and saw a video from that night. I know that was wrong, and I regret doing it. But what I saw in the video changed how I view the situation. In the video, she is dancing very intimately with him. She is grinding and twerking on him, he is holding her, they are smiling and laughing, and there is clear mutual flirting. This context was never mentioned to me when she originally told me about what happened. I was only told about the forced kiss, which naturally made me feel sympathy and go straight into support mode.
For context, this happened before we were officially boyfriend and girlfriend, but we had already agreed that we would not be seeing or hooking up with other people. We became official around two weeks later.
What’s bothering me the most isn’t just the dancing itself, but that I feel like I was given a very incomplete version of the story. The added context changes how I understand the situation and how I might have reacted back then. At the same time, this puts me in a difficult position, because confronting her means admitting that I went through her phone. Something I’m not proud of and don’t feel good about.
So now I feel stuck. Based on the boundaries we had, this feels like a form of cheating to me. At the same time, we were not officially together yet, and I don’t know if I am reacting out of hurt and ego or if this is a real breach of trust.
I am not trying to villainize her, and I know I also crossed a line by going through her phone. I just don’t want to ignore something that genuinely feels wrong to me.
Am I overreacting, or is it reasonable to feel betrayed by both what happened and how it was communicated.