Has she laid eggs recently? I would susupect that she might have ascites or egg yolk peritonitis, both of which can cause a swollen lower belly between her legs. The upright posture in the pictures can be a sign of egg yolk peritonitis and and salpingitis. Some people try draining their hens with a 16 or 18 gauge needle and large syringe inserted just under the skin after disinfecting the skin. It is best done by a vet, but many people do it at home. It may ease her breathing to relieve some pressure. It is only a temporary fix, not a cure, and can result in infection or death. Eventually she will need to be put down or she will die of her problem.