Have you noticed her poo? 
I probably can't help in any way, but I can pray for your hen.  ('Hope you don't think that's too wierd... we love our pets)
We are not experienced chicken farmers, and both of the ones we have now, we rescued within the last 2 years.  
My husband and I are trying everything we can do... to comfort our own 1 yr. old hen.  We have had little sleep lately... and tonight we feel like we are "hospice for hens." We are sort of "on watch"... have moved her inside, and I haven't left her side almost all day. 
Sunny's abdomen is also inflamed and she has been "pooing" what looks like egg yolk with little bits of very dark poo for 3 days (that we know for sure, we brought her into the house). She is very heavy and labors when she breathes.  I have done countless hours of research, coming to the conclusion that she has egg yolk peritonitis.  I posted today, and have had a couple of replies... but no real hope that she will come out of this.  We have no local vet who treats birds, so BYC is our "go to" option, and we are very thankful it's here... 
We wormed her yesterday, and have started antibiotics (penicillin) as of today, but it might be too late for her... we hope not, but unfortunately, she seems to be going downhill fast.  Her comb and wattles turn dark whenever she sits... which is almost constant.  A few minutes ago she got up, looked around and slowly sank back down, appearing to be dizzy... but this time she leaned onto her right side and her left leg extended... with her toes curled.  We thought it was over... but after a few strokes of comforting rubs... she "snapped out of it."  She stood up again a few minutes later... with no problems, but now is resting.  
(We have also lost 2 hens this year, one of those, perhaps to the same problem.) We have been praying for her, for God's mercy, but also for His will.  We've really been blessed by ALL of the chickens we've rescued, but we might soon be out of the chicken rescue business.
I hope you find help for your chicken.  If you are also a praying person... please send one up for Sunny. (and my husband - who has already shed many tears when the last 2 went to poultry heaven... he's pretty edgy now.)