18 month old Maran has stopped laying, gone listless and has pale comb

[princesslayer]

We have three hens. Two are fine. We dusted for red mites about a month ago as we noticed there were some in the coop. One of the Marans has stopped laying, has gone quite listless, sits there all puffed up and with head pulled in and her comb and wattles are pale.

Any ideas?

p.s she is also the stupidest one by far!!