At this age your chickens are figuring out their pecking order, and this poor girl has apparently wound up at the bottom. Make sure she is still able to eat and drink and that the pecking isn't causing her any injurious harm--feathers may get pulled loose, but if they aren't drawing blood she should be fine. Our flock went through the same jostle for position when they were that age, and now everyone gets along peacefully. If necessary, separate your girl from the rest of the flock a couple of times a day so she can eat, drink, and rest. Things should settle down over the next couple of weeks as everyone figures out what their status is.