Lethargic heavy breathing old hen

[Razz Gavin]

I have a hen I believe is around 7-9 years old (my oldest hen in the flock) who suddenly turned lethargic over the last couple days. I brought her inside and she's interested in pecking around and trying food but she won't eat more than a few pieces and her poop is extremely watery and smells terrible. Just today she's picked up this really heavy breathing. There's no gasping or discharge and since she's a polish I can't really tell if there's any sickness from looking at comb coloring. I feel this may be coming on because she's so old, but I'd really like to know if there's a way to make this easier on her. She's never ever been social with people in her life, but now she doesn't want to leave my side. All I can think to give her now is food, water, a bed, and some neck rubs. Any advice on how to help her?