I'm starting to get concerned with how much my girlfriend and best friend are talking. Is that unwarranted?
Why are you concerned about this?
They do most of their talking when I'm not around.
You shouldn't be jealous about your gf having a friendship with your friend. You should put more trust in her.
I'll probably just starting acting like I don't care and leave the room when they talk.
You should actually not care because you should trust both your girlfriend and your best friend.