Is he cheating?

I kinda feel like my boyfriend may be cheating on me. I love him with all of my heart. But I found messages on his phone from a girl that was supposed to be my friend. They both said it was a joke, but I sorta don't believe it. Then drama was started (by me) and she texted him "you better get your girlfriend, before I tell her everything and make her look stupid" I just don't know what to do.