I really need some advice to calm myself down (so sad)?

[258]

short version of the story is that I just found out that my bf and this girl that flirts with him all the time (right in front of me) have become friends on fb for over 2 weeks now... (world ends)
I pretty much feels like rolling on the floor crying, and kicking table leg T_T
He doesn't flirt back, nor spend much time on fb, but I still feel so violated, and I do get mad inside when she would just come between my bf and I then start flirting with him. All I can do was move to the side and not look angry D=
I don't think I want to bring this up to my bf yet, since it has only happened in front of me twice. I don't know what's the best way to bring this up either. I don't even know if I should be this annoyed since I know my bf is a very reliable person, but even that I still feel annoyed, mad and violated....
This kind of mood makes me think that I'm not really mentally mature or healthy...
anyway, share your view about this, or any advice on how you would have handled this.
Thanks!