There are a number of reasons why a rabbit may have breathing difficulty. If there is any discharge from the nose or your bunny is flaring his/her nostrils in an attempt to breathe more easily, your bunny is having difficulty breathing. Please visit a good rabbit vet for a checkup.
If a rabbit begins to open mouth breathe, this means your bunny is gasping for air. This is an emergency! Please take your bunny to a good rabbit vet or emergency centre as soon as you can.
The below website has good information on all kinds of respiratory illnesses that could cause your bunny to suffer respiratory illness if you would like further information.