buff cochin fading feathers, runny stool, brittle/thin eggs, suddenly lethargic!

[djmixalot]

I have a 2 year old buff cochin who has displayed relatively runny, yellow stool since we got her 6 months ago. In the past few weeks her feathers have begun to fade to almost white in some places, her formerly brittle eggs crack upon laying, and she didn't even hang with our three other chickens today. I gave her some yogurt about a week back and her eggs improved. Is this an extreme calcium deficiency or worms as yellow stool will denote? Should I be giving calcium supplements everyday? Is there something else I should be worrying about - should I quarantine her?