Hen attacking older chick

[Silivrenwolf]

I have a hen that about a day ago started attacking one of my chicks.... my blue cochin when I let the chickens and chicks out. She will chase the poor girl and usually only stops because I'm there to break it up. As far as I can tell it's only that one particular chick as I never see her go after the others. They got along together just fine until about a day ago and as far as I can tell get together just fine in the coop.

Any ideas on how to stop this or why this might be happening?