I feel like I'm going to get into a fight.
Why do you want to get into a fight?
Because of my girlfriend, she came out to me as bisexual and I am lesbian
Fighting with your girlfriend won't fix any of the issues you have with her bisexuality. As long as she is still faithful to you, what does it matter if she's attracted to men and women?
I don't know how to talk about it with her and I feel betrayed.
I'm sorry you feel betrayed, but you need to have a conversation with her about it.