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Rapid breathing can often be caused by some kind of stress. You should evaluate the situation and see if there are any new stresses.
Obstructions of the nose, tracheal problems and laryngeal problems can also cause increased (or seem to cause) increased respiration.
If your dog is not on heartworm prevention, he might be showing clinical signs of heartworm disease or other heart problem.
If it continues, you need to see your vet as soon as you can.
Let me know if you still have questions. I am afraid that my answer is a little vague so feel free to ask more questions if you have them. I will be happy to tell you what I know.