It does sound like it's from your Roosters mating behavior. And it's true that you need to get some more hens for him so he divides his time between multiple girls and thus, hopefully doesn't hurt them so much.
You can also try 'saddles' for your hens since you don't have so many. That will protect their backs from him.
You could rehome your Rooster or keep him separate for a while until the younger girls grow up; thus giving him more girls to 'date'.
You will want to keep a close eye on her, if she starts to bleed (and even if she doesn't, the other hens may start to feather pick that area making it even worse. You will then have to separate her and treat the area.