It looks like she has been feather picked. She could be doing it herself or others have done it due to bullying or overcrowding. Does her lower belly seem more enlarged than your other hens? When it is bare it can look larger, but may be normal. If it is enlarged, she might have a reproductive problem (salpingitis) or water belly (ascites.) Does it feel solid or squishy? Is she able to poop normally? If a stuck egg is suspected, a human calcium tablet with vitamin d can be given orally. Usually an egg bound hen cannot pass droppings. Is her crop emptying overnight, or is it full and firm or squishy? What do her droppings look like? What do you feed her? Is she eating? Be sure to look her skin over for evidence of lice or mites.