Poorly Chuck

Hi all,
I have had my 8 chucks for almost a year now and all has been well upto now. Over the last few days one of my chucks hasn't looked well. She initially started to go to bed early (approx 14.30) and her comb has started to get darker. I put some multi vitamins in the water and she seemed to perk up but this morning she seems to have taken a turn for the worse. She came out, her wings have dropped and she was falling asleep on her feet. She eventually dropped down and looks like she is trying to sleep. She was eating ok yesterday both corn and some veggies but i am now at a loss. She is about 3 years old btw. The other birds don't seem to be affected and are doing their normal thing.
Any ideas anyone ?
All help greatfully received.

Thanks

Tyke 

Vat couldn't see her until Monday.
Gina died this evening 6pm. First chuck of mine thats had died.....no like it much.

RIP Gina x

so sorry, hugs xx