These are very concerning clinical signs. Heavy breathing and lethargy can be a primary lung problem, a cardiac or circulatory problem. Any of these can result from serious disease. I would definitely recommend having your pet examined by a veterinarian. Your vet will take a full history of the problem and during the physical exam they can listen to your pet's lungs and heart and evaluate your pet's pulse and circulation.
Why is my dog breathing heavy not active and loss of appetite
Why are you asking this question and not on your way to the vet,he needs to be able to breathe in order to live...good luck