How/Should I bring up a previous problem in our relationship?

My girlfriend went on a mission trip in July that was 12 days long. When she came back she was acting weird and protective of her phone. I soon found out that she had been talking to one of the guys on the mission trip and that she had taken a few picture of him that were uploaded to Facebook. Also the day after she came back they hung out and went on a run/walk together. Since then she's deleted his number and stopped texting him back.

I've been thinking recently that I was too nice when this happened. I don't feel that I got my point across of how upset and hurt I was that this happened. I want to tell her that I'm still hurt by it but I also don't want to restart a fight over something that happened 3 months ago. Any suggestions?