Welcome to BYC. Is it very cold where you are located? I would isolate this hen in a warm place and watch her for any nasal drainage, swelling around her eyes, sneezing, rattles or wheezing, or other symptoms of a respiratory disease called mycoplasma (MG, CRD.) This disease can make carriers of your flock if they are exposed.
I have a hen who started getting bubbles in her eyes whenever the weather is below freezing. She has no other symptoms, we have had no MG in our flock, so I'm thinking she has something wrong with her tear ducts. Here is a good link about mycoplasma gallisepticum and other respiratory diseases: http://edis.ifas.ufl.edu/ps044