My closest friends boy best friend ?

You shouldn't keep this to yourself. Your best friend trusts you and if she found out you've been keeping something to yourself, she's gonna be upset and feel like you don't trust her. If you tell her, she won't be as mad/upset than if she found out herself. Besides, it wasn't all your fault you are a naturally flirty person. If you tell that to your friend, she'll probably understand and won't get too mad at you. Hope this helps!