Coughing or sneezing on its own is not necessarily a sign of illness or infection. It could just be a reaction to the colder temperatures or dust in the atmosphere, if for example the birds have been staying indoors more due to the bad weather and have been scratching around in dusty conditions inside the coop. If this is the case then you have nothing to worry about - an odd sneeze here or there is perfectly normal in a flock, especially during colder weather.
However, when you look closely at your birds do any have discharge from the nostrils or bubbles in their eyes? This is a more reliable indicator of respiratory illness that might require treatment. A sick bird will also tend to hunch up, not eat, and not socialize with other birds.
A bird displaying these symptoms is more worrying and should be investigated - if possible by finding a local vet who will take a look, even if poultry is not their speciality.
As a new chicken owner who worries about their birds (and we've all been there!) it is very tempting to reach for antibiotics at the first sniffle, but if you are considering this option I would advise against it until you are certain that your birds really have an infection that would respond to treatment. Otherwise you risk creating a resistance in your flock that could have more serious repercussions further down the line. However, if several birds start looking sick in quick succession then you will need to react quickly before any infection gets too severe.
Fingers crossed for you and your flock that it is nothing serious.
