Chicken Help!! Any suggestions?

I have a chicken, probably around 2-3 yrs old, we got her and some of her buddies on the 4th of April. About a month ago, we introduced some new young birds, gradually, and now everyone is co-existing well. On Friday, I noticed my big girl Biggie, had her comb was discolored and laying on her head (usually is erect and a bright red), she seemed very slow, delayed reactions and very careful about where she stepped. She's way out of it. She is not eating, depressed like, stays to herself, not interested in her free time (not even when everyone sounds the worm alarm and they go nuts) she'll just stand or lay there, not interested. There has been a few times where I see her yawning (like she's trying to dislodge some thing, but she's not gaging or choking) and she occasionally has a goober of drool from her beak. Her breathing is rapid, and she has some runny stool, but its not clumped on her bottom and she is not herself at all. The young birds were de wormed 2x prior before introduction, and the big girls were de wormed 2x before the introduction and as an entire flock they were de wormed after introduction. My husband cleans the entire coop ever 2-3 days, they are fed crumble, oyster shells every day, and corn 1 to 2 x a week thrown out in their paddock where they have free time. They get free time in a controlled setting, and are allowed to forage for bugs and on grass. I don't think she is laying, she's hardly walking or eating. We separated her to prevent the other girls from picking on her. Any suggestions. I was leaning towards coccidosis, however, labored breathing doesn't fit into the symptoms, and sour croup, but she doesn't have a smell? Any suggestions. I've tried all the vets around here, and none of them see chickens and yest we are in a rural area! ={ They suggest butchering her to prevent contamination of the rest of the flock.... ok if I need to , but I will not do that yet, because I don't know what it is and no other birds are displaying any of her symptoms. Thank you.