I'm a 18-year-old male seeking some perspective on certain behaviors of my 19-year-old girlfriend.

I apologize for any spelling mistakes; English isn't my first language. My girlfriend and I have been together for just over a year and have faced some minor issues. Recently, I've been struggling with communicating with her friends, as I don't have many friends myself. When I tried to engage with them, it didn't go well, and she ended up deciding to end things, citing my insecurity. We worked things out and chose to stay together. In my country, we have a major celebration similar to a carnival, but we weren't able to go together. I trusted her not to cheat or dance with another guy. However, she later mentioned that she was sleeping next to a male friend, which felt odd to me. She had also been distant and, at one point, told me to shut up and go to sleep because she didn't want to talk. I've been feeling really down lately and even cried myself to sleep several nights. I skipped the carnival because of how I was feeling. Today, she saw an old message on my Instagram (she has access to my account) from someone I used to talk to but stopped messaging at her request. She reacted strongly, calling me names and saying she should have cheated on me, claiming I’m worthless. Despite being a generally affectionate couple, when she gets upset or has a breakdown, she can lash out like this, and it really hurts me. She has been my first everything, and I truly don’t want to break up with her. Why would she react this way? Is she possibly cheating, which could explain her behavior?