We need to know her age, breed would also help, poop color and consistency, color of comb, injuires, wounds, is she lethargic, laying down, eating, drinking....etc...Anything you can tell us will help to diagnose her.
Looks like aggression to me. Hens are fighting. You need to put something on this wound, like blu-kote or even neosporin, (without the pain killer). You need to find out which one of your hens is being aggressive and separate them out for a couple weeks. You can also use pinless peepers on the ones being bully's. Never let birds be bullys to each other.
Many times bullying starts because they are cramped in. Chickens need enough personal space so they don't fight with each other. At least 5 square feet in the coop, 10 square feet in the run. More room if you can afford it.
And give them things to do...hanging cabbages or veggies, seed blocks, flakes of hay...just something to do other than pick on each other.
So for now, you need to keep this injured hen in a cage within the flock so she can heal for a few days. Spray the wound with blu-kote or apply some antibacterial ointment. But you will need blu-kote to hide this wound and skin for a while til the feathers grow. You can get blu-kote at feeds stores in the horse section. Spraying this area will stop the others from seeing it and picking on her.
Then you need to figure out where the aggression is coming from and who is doing it. Separate them out for a few weeks or use pinless peepers on them.