Suspicious

Last year 2018 my boyfriend made the dumb decision to take a girl at his school to his college formal (didn’t ask me at all) because she was “antisocial and her friends wanted her to go out more”. After that fight we were ok until this past December 2019 I found flirtatious messages between them. We fought for a long time about this bs and days later it was settled. Deep down I cannot stop thinking about this whole situation. He’s not the type to cheat but he’s done shady things to make my trust for him decrease. Idk if I’m over reacting with anxiety or have a true bad gut feeling. Also, We’ve been together for 3 years...Helpppp