Sorry to hear your hen isn't up to par. I hope I can help.
Do you mind posting pictures of her poop? I know it sounds gross, but often the feces can be good indicators of a disease or other problems.
The heavy breathing would seem to imply a respiratory infection, but I have had chickens injured before who breathed heavy because of the stress and pain and not because of any lung diseases.
Her stop in laying is probably directly related to whatever is wrong with her. Hens that have diseases, injuries, or parasite infestations will often cease laying if their bodies are stressed enough.
Like I said, please post poop pics, and let us know anything else that may be helpful. If you have an avian vet nearby, it might be a good idea to get her checked out. Also, try calling your local vets...I didn't know that I had an avian vet in my area until I went to our normal vet and found out that they have an avian vet as a member of their staff. The price for check-ups has been very reasonable.
~Gresh~