It sounds suspiciously like a tumour I am afraid. I have lost four of my ex-batts over the years to tumours which older birds are prone to.
They tend to take on this stance when the tumour starts getting too big for them. If you pick up the hen, her tummy will feel fairly solid which is a sure sign.
The first one I experienced I probably left too long before taking to a vet, having tried the warm bath, etc thinking she might be egg-bound.
How are the colour of her wattles and comb? In its advanced stages, they may start going purple and she will become disinterested in food. If she is still eating OK and running around, then you will be OK leaving it. Once her quality of life really becomes affected, she will have to be euthanized.
I hope she is just egg bound and can be treated by the vet but I hate to say it doesn't sound good.