My hen sounds congested.

What are your coop conditions like--are they dusty or have good air ventilation, or is there any wet or moldy areas, or a strong ammonia odor? Things like that and dusty feed can cause sneezing and respiratory symptoms, but the one hen may possibly be suffering from a respiratory infection such as infectious bronchitis, MG, coryza, and others. Sometimes stress, such as cold weather or other stressors can bring on symptoms, since most of these are chronic carrier diseases. Some diseases are viral, some bacterial, and some fungal. Tylan 50 injectable sometimes is used to treat many of these conditions, or to prevent secondary infections. Here is a good link to read about those diseases with symptoms, to compare what you are seeing:
http://edis.ifas.ufl.edu/ps044