My chicken has a hard time breathing!

I have almost 5 year old Light Brahma hen. Brahmas are big birds, and this is no exception for her. She isn't the really active type, and she sits down a lot. I live in San Francisco, and so its foggy and has mostly cool weather. But she still gasps and wheezes when she runs even though it isn't hot or anything like that. She also isn't a very productive egg layer.The last time she layed an egg was last week (thursday) and today is Friday (of the next week). Sometimes when she sits down, you can see her body move up and down and you can see her beak move slightly open and close. Her poop is like a light brown and runny type of poop, but she also has normal looking poop. She still acts okay, clear eyes, preens, basically normal chicken behavior besides the wheezing. She is eating and drinking daily and normally. Is there anything wrong? If so, what could I do? Thank you very much for any advice you might have.