Does the bag feel soft and squishy like a water balloon? Or can you feel an egg in there? Have you had her isolated long enough to know when she last laid an egg?
Reason I'm asking is because I have a hen who gets like that every three months of so - she suffers from Ascites (filling with water) and her backend becomes a big water balloon but she still lays eggs. So I use a large syringe with a small needle and literally drain the water out of her every couple of months or so. It comes out absolutely crystal clear and looks like drinking water it's so clear. After a few pokes she starts to drain on her own, leaking from the needle holes. She will drain out about a cup of water. Then she's o.k. for another three months or so, lays eggs, free ranges, perfectly healthy in all other respects. This has been going on for several months.
It wouldn't hurt to try and drain out a needle full of whatever you can just to see if it's clear water or yolky looking. If it's discolored, thick and/or yolky looking it could be that she's an interal layer and I'm afraid there's no treatment for that short of hysterectomy.
Good luck
