Is my bf cheating?💔💔

Guys how do you know if your partner is cheating, we have been dating for 3 months now and honestly he is acting a bit cold with me for the past week, I found out a girl’s message on his phone on Saturday and today I was going through his ig, I found that same girl commented on his last photo which never happened before. I don’t know what to do I feel like crap... please help 💔😭😭 I really like him and he was the guy I lost it with because I knew I was ready... I don’t want to loose him