It would be hard to know if it is the beginning of signs of internal laying, salpingitis, or ascites. Or if she is just a little fat there. I have had a few hens with huge lower bellies that have lived a couple of years after developing that, and I usually do a necropsy to check on what is in there. I don’t intervene much, but just try to keep them eating and able to run around with the flock.
Some hens that have had egg laying problems just quit laying altogether, and later may die of unknown causes. I necropsy everyone now since my chickens are all over 6 or 7. Usually when they are sick, they separate themselves and act lethargic, not eating well or sometimes not roosting. Checking the crop for emptying overnight is always good to keep tract of in a sick chicken. Crop problems may accompany laying disorders.