What is going on with my bird?

[Annasg]

I have a Plymouth Rock hen that looks seriously overweight. I know Plymouth Rocks are big birds but this bird worries me. I noticed today that she seems to be having a hard time breathing and she has a dark streak down the front of her chest. She also has feathers missing right under her butt. Can anyone tell me what is going on with her? I've never had these problems before, although, it has been a long time since I kept chickens. Please, someone, help me out here?