It her belly enlarged? Fluid (ascites/water belly) or internal laying could make her breathing more labored. If she has ascites, a common problem in older hens, then there is not a lot you can do about it. You could drain her belly with an 18 gauge needle and large syringe from your feed store, and if you get yellow to dark amber fluid, that will confirm it. Draining occasionally can relieve pressure and ease breathing p, but won’t cure her. It also may cause infection or rarely, death, so not without risk. I usually don’t drain, letting nature take its course. When a hen dies, you can perform a necropsy at home, and look for ascites, salpingitis, cancer, or tumors. It is a good way to learn about what was going on. How does her crop feel?