Girlfriend referred to me as "her bitch"?

I found out through a close, trusted friend that my girlfriend of a year keeps referring to me as " her bitch". That even before we started dating and were just talking, she was apparently bragging to her friends and to my friends that she's going to "make me her bitch" and was showing them my messages to her. Why would she say about a guy she wants to date "I'm going to make him my bitch"? What does that mean?

Updates
+1 y
Is this bad? Is this a red flag?
Girlfriend referred to me as "her bitch"?
Post Opinion