Doesn't sound too good.
Is she near her flock so she can see them?
I think I would cage her in the coop/run if possible, that way she's not so isolated.
Could be a number of reasons why she's starting to decline, but do the best you can to get her hydrated and eating. See how it goes.
Just keep with what you are doing.
I'm just not a fan of giving any type of pain relief. That may sound cruel, but chickens will self limit activity based on pain level and they seem to heal more quickly that way. Masking pain is not always best practice.