I dont know what to do with this.

So basically im a 16 male. And i have a girlfriend. She loves to talk to other guys, but gets mad when i talk to my friend who is a girl. Like not just mad, i mean like Angry in a type of way where she wants to kill me. And i’ve known the girl for 3 years. And i got my girlfriend like 9 months ago. Today there was a girls basketball. And i love basketball so i go to the game to watch it. When im there i meet my friend (the girl) and my girlfriend. After idk about 1 hour later a guy that is 2 years older than me, starts talking with my girlfriend. They got so close that my brain couldn’t process it. They are friends yes i know, but she doesnt even talk to me that much. I always try to be around her. After a while the guy says that he likes a girl in his class. And my girlfriend is friends with that girl. So she was suprised. The guy tells her why he likes the girl, but before he made her promise that she wont tell anyone. She promised. When i got to the railing to see that basketball i jokingly said “I dont promise to not tell the girl” When i say that the guy starts choking me against a wall, but not like really choking like easily pressing my throat against the wall, then my girlfriend says he can hit me also, so he does. She doesnt try to stop him. After it all ends my girlfriend gets mad about me talking to my friend. I know this may be a red flag, but i like her, so help me.