AH, could be the fumes then, though I would expect more in the flock would be affected, but maybe as the others said, it may have drifted to something she ate too.
I did have a hen that laid few (not many) eggs and was always nipping the other chickens (including the roosters) tails, so much they had nubs instead of tails when she was around. I think she liked the bloody pinfeather taste (ewww, I know). Your hen is not quite a year old also, and I have observed some hens around her age in my flock start to throw their weight around and establish themselves higher in the pecking order too. When they are like that they double down on any hens that may seem to challenge their status during that time.
