My girlfriend, since we met, I just sent her a message and she read it and didn't reply anything. What does that mean?

Since you choose to associate, don't doubt each other. She didn't reply to your message. Maybe she is busy, or maybe she wants you to call and talk to her. The most important thing between two people is mutual trust. If you don't understand, you can talk to her, but not too directly. Please accept it. Thank you.