AITA for not being comfortable with my girlfriend having a male friends number that’s also in a relationship?

I am 19. My gf is 22. We live together, and she has a friend that is also a man. Let’s call him Guy. Guy is also in a relationship with someone. Let’s call her Girl. My gf is friends with Girl and has been for almost 4 years. My gf and Girl used to live in a dorm. Freshman year of college is when Girl and Guy met each other. Guy started coming over to their dorm a lot and getting to know Girl. They are happily in a relationship together, and I’m very happy for them as well. My gf, Guy, and Girl all have a group chat together. My gf has Girl and Guy’s number. I don’t know how often she texts Guy or what she texts about with him, but I hope nothing crazy. Anyway, I’m going off topic, but I’m sure this is just me being insecure. I only found out recently that my gf had Guy’s number. I thought my gf just had Girl’s number because they’re mostly closer since they lived in the dorm together. Guy just came to hang out with Girl. Recently, I made a mistake when over Thanksgiving, my gf texted me about how Guy just texted her. He said something along the lines of me and my gf should hang out with him and Girl. When I got home, I didn’t outright tell my gf this made me uncomfortable. I just lightly implied that it did. My girlfriend was kinda upset. I said I didn’t know you had his number. She just said, “Well, we all knew each other for a while, so it just happened.” I didn’t want to continue the conversation, so I moved on to a different topic because I could tell my gf was uncomfortable talking about this subject. Also, I was kinda high when I brought this up with her. So I probably said more than I should’ve, but whatever. I love my girlfriend, and I already know I’m kinda in the wrong here anyways. I just wanted to hear your thoughts on this.