That's very normal behavior for newly introduced chickens. Looks like she is near point of lay. She may grow a little bit more.
The best way to go about introducing a new bird to the flock is to keep her separate, but nearby. This allows everybody to get used to each other without putting anyone at risk of injury. After about a week or two, you can bring in one of the more mellow, easy-going flock members to be her buddy. Once they are accustomed to each other you can allow supervised run/free range time with the flock. If that goes well, you can start cooping everybody up together.
Introductions are something that needs to be done gradually. You can't just throw a new chicken in the pen and expect everybody to get along from the start.
Edited by junebuggena - 1/7/16 at 4:29pm