I found out that my girlfriend is still in touch with her ex-boyfriend.

Girls, give me some advice, I'm really struggling right now. I caught a message on my boyfriend's phone where he was being pretty close with his ex. When I asked him about it, he said they were just friends and that they've known each other since childhood, but I really don't feel right about it. Do you think there could be an explanation for this, or should I just end things?