Why did my chicken die?

Hi
I am a new member hoping to get some advice on keeping my chickens. I purchased 3 hens in July this year (all POL)- 1 black Rock, 1 Maran Cou Cou and 1 Warren and up to last week they were all fine.

Last Wednesday my husband noticed that one of them had runny poo-pasty white and watery, but they all seemed to be OK. Then on Saturday morning he found the Maran Cou Cou near death. We examined her to see if she had an egg broken in her vent or anything and she really smelt, plus we found all the feathers missing from her breast down to her vent and she was very skinny. She was the most difficult of the 3 to handle so we had not noticed anything until then. There was no sign of parasites and the area was not red. She rallied a little when we gave her water with a syringe but sadly died Saturday evening. Does anyone have any idea what killed her.

If it was just the diarrhoea we would have thought it was due to something she ate but the missing feathers are a mystery. I wormed them 2 months ago and put cider vinegar or poultry drink in their water regularly. They are fed on layers pellets with the odd treat late afternoon-meal worms, corn, or veg etc.

At the moment the remaining two seem OK are are laying.

I would appreciate any advice.

PS She wasn't egg bound and I have now wormed the other two who seem alright and are continuing to lay.