Altho to the sounds of it, she was slowly deteriorating and in that case it wouldn't be SDS. Are there any more symptoms? like was she standing up properly? was she still eating and drinking?
Sounds like it could possibly a case of SDS (Sudden Death Syndrome) "Sudden death syndrome is a condition in which apparently healthy chickens die suddenly from no apparent causes. There is usually a short wing beating convulsions prior to death."
Or Possibly She was Egg-bound
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