losing a chicken
It sounds like she may be suffering from internal laying, egg yolk peritonitis, or ascites (fluid in the belly.) How old is she? Has she been laying eggs lately? If her lower belly is full and tight with fluid, you could try to withdraw some of the fluid with an 18 gauge needle to help her breathing. Antibiotics can be used to prolong the life sometimes in hens with EYP. A vet could probably help with treating her, or helping to put her out of her misery.
Here is some reading about egg peritonitis and ascites with a photo showing where to drain fluid: