Pet chicken suddenly not laying..lethargic..expells a yellow yoke like substance.

[2976]

I only have 4 chickens and she is my strongest and friendliest. since I have only had these ladies for about a year, I have no idea what could possibly be wrong. All I know is that I want to do whatever I can to help her feel better. They are locked in a coop at night and free range during the day. The other 3 chickens are healthy. I did check for egg binding that I read on this site... but I don't know what else to do.
Any help would be appreciated.

 

[Laya egg]

I am sure you read about egg peritonitis. I thought for sure that that was what one of my hens had, but later decided it was stomach bile since she had stopped eating the same day I noticed the yellow substance. After an Necropsy I had less answers than I started with, but became more convinced what I saw was bile. Have you considered worms of some sort? Is she eating or drinking? Is her comb pale and droopy? Is there waste adhered to her bum?

 

[2976]

First off... thank you so much for replying. She is drinking and eating fine. I gave her a little antibiotic and deep cleaned the coop. The other 3 chickens are great, no sign of any kind of illness. They do free range so maybe she ate something that isn't agreeing with her. She is normally the leader of the pack so I hate to see her down and out. It is almost like she is "sort of" developing an egg, but not quite. Her egg shells have been very thin for the past 10 days or so as well.