The current boyfriend sent a message to his ex-boyfriend saying: She is already my girlfriend, please stay away from her. What does this mean? Your current boyfriend doesn't believe me?

Declaring sovereignty is not necessarily a question of distrust. You have to get along with it for a while before you know this trust problem. Just from texting your ex, it depends on whether your ex sent you a message or did something jealous. If there is nothing, it is probably to show off and be narrow-minded.