Bad/sick chook Please Help!!

[GosoChooks]

We have four chickens, but we seem to have a problem with the leader. She has a really red bottom, this doesn't seem to aggravate her she also doesn't scratch it. It started with just a bit less feathers, the problem got worse so we got some insect powder and followed the instructions. We left it for a bit to see if it would help, but the problem just got worse.It looks really bad but when you touch it she doesn't seem to worry about it. What could be the problem?

 

[GosoChooks]

An update on the same chook, we recently had a fox attack our chickens, I came out to find sunny (the chicken) in the jaws of the fox. luckily It let her go and she seemed fine until just then Sunny dropped on her side. Another chook started pecking her, I ran outside and picked her up, she is loosing alot of her feathers under her chest aswell. She is also not using one of her feet.

Please help!!