This could be a respiratory issue which is very common in chickens. They should be treated immediately with a medicine (not sure of names, but you can ask your vet or pet shop). To counter respiratory diseases in your flock ensure their house is a dust-free, dry environment. Ensure adequate ventilation and use straw as bedding - it is by far better than any other bedding and on no account use sawdust!
Alternatively, your poor hen could have gapeworms, which are not common but do occur. They are nasty parasites that clog up a bird's respiratory tract and stop her from breathing properly so she 'gapes' in an attempt to breath. These are countered with wormer vaccinations which are available from the pet shop or you can use herbs or garlic generously in their diet. All animals must be wormed regularly or parasites will build up inside them.
To boost overall flock health and repel worms you should grow a herb garden (definitely include garlic) and dry your herbs to add to your flock'so feed over the year. For advice on gardening and poultry check out fresheggsdaily.com
