But I have had chickens get withdrawn from the flock when their buddies have died. A good example of this is a 3 1/2 yr old EE roo I have who just sulks around the yard doin his own thing. In the pecking order, he'd get his butt kicked, so its better this way. If you plan on getting her a new buddy, I'd suggest one that is a similar age, as well as being of a similar temerment. Birds in my assorted flocks have done this, ones who didn't fit in a catagory with others would simply roam alone.
I have had a Buff orp X do the same thing that you described: Head down, feathers puffed, wings spread. It had to do with her wanting to go broody, but she was being used at a childrens camp so there was no place for her to set.
Regarding insane birds, I'm pretty sure I had one that developed a brain tumor that caused her to behave oddly. She would spin around in a circle and honk loudly (barred rock.) If you stopped the spinning, she would be normal for a bit. I was kinda glad when a fox got her, the spinning was getting in the way of normal activites like eating and ranging with the flock.