I agree. You are overreactiong a bit too much about everything right now.
You did have the right to know why she was talking with him for so many hours. Any man would get jealous over it.
At the same time, you shouldn't demand she stopped talking to him. You could just suggest that she limit herself or something similiar. You are her boyfriend, and if she gives another male more attention than you, somethings up.
But you could just chalk this up to her inexperience in relationships.
Just giving my cents.