What sort of bedding do you use in the coop, Fuzzyfelt? If this is at all dusty it can affect their breathing. Also, how much ventilation is there at night? You say you lock them in the coop, so they are then reliant on vents for air amd it's essential that they have lots of it, don't worry about them getting cold.
If you know neither is a problem, then yes, she has a respiratory infection, which should respond to antibiotics if you can take her to the vet ASAP. It 's quite common, and with prompt treatment there's less chance the others will catch it - so long as the coop isn 't stuffy overnight.