why do I have trust issues with my boyfriend I've had for a year so far...

I get really pissed off when my boyfriend talks to other girls 😩 especially when he has me, he shouldn't need too. I'm fine with some girls because they're apart of our friend group and I know they wouldn't do anything to me. Anyways I was staying at my boyfriends house and I noticed he started a streak with this girl on Snapchat. It's nothing big.... but she's not apart of our friend group, I've never seen him talk to her nor did I even know they were friends... I sound stupid. I know. But like he wouldn't Snapchat her if I was watching his phone screen. If I had my back to him he'd reply to her but when I turned around he wouldn't dare to even unlock his phone and go on Snapchat. It worries me and makes me over think but at the same time I know and tell myself he wouldn't do anything to hurt me like that. 
Someone please help me figure out how to get over this stupid jealousy over thinking feeling πŸ˜“πŸ˜”πŸ˜”