Deep/Heavy breathing chicken?

I'm at a loss here.

Our light Sussex has started to breathe deeply - but not panting as if she's just ran a marathon.

Which ever way it is, she opens her beak when breathing, either when breathing in or when exhaling, yet non of the others are doing it. 

I've done all the checks suggested on here - vent, temperature, crop and 'rattling' etc.
But I still can't figure her out. 

Any ideas? 

when was she wormed? gape worm could be the problem as Sassy suggested also take a look in her throat to see if you can see a worm (often you can) or to see if she has anything stuck in her throat such as a small twig, piece of bedding etc..

First of all check her throat just to see if she has anything causing an obstruction.
Check the bedding isn't wet or dirty as this can cause breathing problems (the ammonia given off can affect their breathing).
Check her crop to see that it isn't hard or squishy. It should be empty in the mornings and then fill up over the day but you should be able to feel any grains in there.

Just an update, she's not doing it anymore! 

Must have been that bit of warm weather we had putting a bit of heat into thier coop and bedding...