don't know what to do

Okay I really don't know what to do😭😭 I've just found out my bf has been messaging another girl and commenting on her Instagram and vice Versa. He (eventually) told me that that was all that was happening but I don't know if I should believe him. I checked again on her comments and another girl out "Jake you look unreal😍" like wtf?? I'm on his Instagram profile and they're commenting that. He began talking back to me quite disrespectfully but began apologising. Someone help😭