Unwell Chicken - advice please

Hello - a new member to the site. I have 6 chickens - but yesterday one of them (a Fenton Blue) has become very listless, and appears unwell. I have isolated her as she seemed to be getting pecked by the others - and this morning she has definitely been eating and drinking. But she still seems very unsteady on her feet, and weak. Some signs of diahorrea .... and seems to be holding her wings out. Anyone any idea what this might be and suggestions for treating her?

Thanks
Ian
Bath

Thanks - unfortunately she died overnight beforre we had the chance to take to the vet. Not aware that she had been stressed - so think from other reading I've done that it was some sort of lung/bronchial imfection? All the rest of the chickens still seem healthy.

I am so sorry, unfortunately with chickens it is hard to spot somethig is wrong untill it is to late they hide things very well.