Boyfriend acting shady

I had my boyfriend phone and noticed he messaged some random girl that I know for a fact he doesn’t know and said “ I’ll play fortnite with you “ which I find very odd because how did he get her Snapchat. Why randomly message her . Like I know for me when guys message me I don’t talk to them. And now he’s all pissed off and saying I’m not fighting he had done this shady stuff before but promised he’d never do it again