Help! Please can you help?

[Drumbeater]

One of my chickens is looking under the weather. I noticed it first last night when I went out to give them their evening corn ration. She was standing still, with her head tucked down. Her comb and wattles are very dull and dark red. Her eyes look fine, the nictating membrane is normal. She will walk away when I to towards her; she doesn't appear to have any mobility problems. She has no discharge from her nostrils, and she is not panting at all. I've given her a check over and there doesn't seem to be any infestation of parasites; I treat the hen house regularly with an anti mite spray.
She is an ex-battery hen, and I've had her for approximately 6 months. She is kept with 9 other ex-battery hens, and 6 cross breeds which I have raised from chicks.
Has anyone any ideas about what might be the matter?

 

[NEWTODUCKS]

Is she laying ok?

 

[Drumbeater]

it's difficult to tell because she's in with the others.

 

[NEWTODUCKS]

I asked about the laying as I lost one to egg peritonitis which is when the egg goes the wrong way into the abdomen. This typically occurs after moulting when they recommence laying.
If it's that then it's usually terminal.
Hopefully someone else may have somemore ideas.

 

[Drumbeater]

She's eating - not as voraciously as usual, to be honest - but she is making the effort. She seems a bit weak though and not as energetic as usual. She hasn't moulted recently.

 

[Marigold]

When was the last ime you wormed them with Flubenvet? If she has a worm burden it will pull her down even if something else is also in the picture.