My Girlfriend is mad at me..can anyone tell me why?

The other day I was visiting my girlfriend at school. She left for class for a couple hours so I was just in her room by myself with some time to kill. There were some notebooks lying on her desk that I assumed were just notebooks with notes in them. I just casually flipped through them and realized one of them was a diary. It was atleast 1 or 2 years old and all it had written in it was standard diary stuff…boys, hookups, roommate issues stuff like that. I stopped reading because I felt a little weird doing it.

When she came back, I made the mistake of teasing her a little bit about it. Honestly, it was good natured ribbing in my eyes because that’s the kind of relationship we have. But she FREAKED OUT. She honestly acted like I cheated on her or something. I apologized and admitted that it was wrong of me to do, but at the end of the day, it’s just a diary and she’s acting like I’m the worst boyfriend ever. Any advice?? Oh, and btw I’m 22 and she’s 21