Girlfriend tells her best friend every time we argue

Recently I just found out that my girlfriend tells her best friend every time we have an argument, or whenever I end up upset at her for something she did.

For a little bit of context, I was on FaceTime with her brother and her best friend texted her and I told her that she had a text from her and she said just reply, and it’s okay if I read their convo.

Well, reading her convo I come to find out that she told her best friend about our argument last night, and how she told her best friend every negative little thing about the argument, rather than how my girlfriend confessed that she was in the wrong and ended up making up. Also going off of the text that says “he always gets upset about stupid little shit, you always text me and tell me that he’s mad for something”. Seeing that her best friend is obviously on her side, they(her best friend goes by they/them) just sent a bunch of relentless, rude and snide comments about how I’m wrong and how I shouldn’t do so and so and how she should want more for herself and blah blah etc. I admit, I’m very temperamental, but I’m also big on communication and immediately talk it out whenever I feel upset, sad, depressed, whatever the mood I am in, I always talk about it. We don’t fight a lot, in fact this was our second REAL argument and we’ve been together 7 months strong. We don’t bicker, or any of the sorts.

On my end, I have never, in any of my relationships(this post might give me a little hypocrisy) gone and told anyone about what’s cooking or brewing between my significant others and I, I have always been about private relationships and being discrete, what happens between us as two entities, stays between us. I can’t shake the feeling but to be a little upset and hurt by this, but is there a chance she just doesn’t know about privacy in relationships, or does she just not care? I feel confused and a little frustrated about this, because I feel as though a privacy boundary was overstepped.

What should I do?