I agree with Spotted Crow...based on this limited information, it's probably just her crop. It definitely doesn't sound like pecking to me.
How long have you had this chicken and how long have you had your 12 week olds? Is she the first young bird you've had? Sometimes their crops get really big and it can be scary if you haven't seen it before! Take a look at your "big girls" after they eat and I'll bet you'll notice the lump on the right side of their chests.
I'm raising my first chickens this year so it's not like I'm an expert, although I did stay in a Holiday Inn Express one night a few years ago.