If this swelling is just below the vent area, it might be from a reproductive infection. Now, please regard that as extremely tentative and in no way a diagnosis. In fact, treating her with an oral antibiotic and having her improve would be more of a "diagnosis". (It's called trial and error or throw stuff at the problem to see if anything helps.) You can try her on an antibiotic and see if it makes a difference.
As far as being bullied, it's common for a sick chicken to be bullied. I have a hen, herself currently being treated for an infection, who goes into an absolute rage when she sees another hen in the throes of wry neck, attacking her without reserve. Humans have nothing over chickens when it comes to persecuting other individuals for differences.
As far as being bullied, it's common for a sick chicken to be bullied. I have a hen, herself currently being treated for an infection, who goes into an absolute rage when she sees another hen in the throes of wry neck, attacking her without reserve. Humans have nothing over chickens when it comes to persecuting other individuals for differences.