Significant swelling around the abdomen, ime, points to reproductive issues. Which one, it would be hard to know unless you lose her and a necropsy is performed.
If the abdomen is filled with fluid, sometimes draining the fluid can give them some relief for a short period of time. Draining...
Welcome To BYC
Does this hen still lay eggs?
For a lice and mite infestation, I suggest that you go to a feed store like TSC and get a Permethrin based poultry dust or spray and treat all your hens and their housing. Repeat treatment in 7 day intervals.
Do you have some photos of her poop...