Chicken seems to be 'contracting' when laying down?

[azygous]

I have some very old hens and a couple are overweight. They do appear to breathe heavily in a rocking motion. Otherwise, there's nothing wrong with them. Just old.

 

[DellaMyDarling]

Could she have a partial prolapse happening? Explains the possibility of constipation.
Knowing if she is still laying would be helpful. This could be figured out by crating as well.