Is she pooping fine? Is it coming out normal? Have you washed around that area with warm water + a wash cloth? It looks like something is wrong with her vent, but I'm not sure yet because I need to know more of her actions currently.
I washed her bottom with warm water n soap it is a little raw, her vent area is a little swollen but it isn't blocked. I have separated her from the two others and she has her own space, food & water. I will put some triple antibiotic on it and am feeding her 20% protein, some yogurt and oats - she looks thinner than the other hens - but she was also getting picked on.