She may have a reproductive problem called salpingitis (internal laying of egg material that solidifies known as lash egg material.) Or she could have ascites (water belly) which is common in hens with liver problems related to reproductive disorders or cancer. Keep her area below the vent clean if it gets droppings on it, since flies laying eggs (flystrike) can be deadly.
Water belly may cause labored breathing and a blue comb. If there is fluid inside the belly, sometimes removing some with an 18 gauge needle inserted under the skin of the lower belly can give some temporary relief. I have had several that had water belly when I did a necropsy, but I only have drained one, since she had those issues of labored breathing. As long as they are getting around well, I do not intervene.