Hmmm difficult one
...have you wormed her recently?
If so, then maybe try a bit of poultry tonic or apple cider vinegar (livestock one) at a ratio of 20ml to a litre, using a plastic drinker, otherwise it stains the plastic...
Keep her warm and if she doesnt improve within a week or so, its probably time for the vet....
Especially as its been on since Jan,
I presume you know her quite well and what tickles her fancies foodwise so keep her eating those things to give her a bit of weight. If you dont then most chickens cant resist a few handfuls of mixed corn or watermelon