Can you give some mire details about your hen including age, and if she has been laying eggs recently? She sounds like she could have ascites (water belly) which is common in some hens with egg yolk peritonitis or other reproductive infection or cancer, liver disease, or heat failure. There also can be hernias there, or enlargement from internal laying egg masses. Does it feel hard, spongy, or tight like a drum? Do her droppings have any yellow urates or fluid in them? If she has ascites, some people or a vet will withdraw some fluid from the lower belly occasionally to help relieve pressure and allow them to breathe easier. An 18 gauge needle can puncture the skin and the yellow fluid will drip out, or a large syringe may be connected.
