Is My Puppy Sick If He Is Rapid Breathing And Opening His Mouth Wide And Sticking His Tongue Out?

What are weather conditions of your area? If your dog is breathing heavily due to hot environment or after exercise then it is normal. Otherwise medical attention is necessary. Most common causes of heavy breathing in dogs are

1. Pain
2. Nervousness
3. Heart diseases
4. lungs diseases
5. Pneumonia
6. Fluid in lungs
7. Trauma
8. Bleeding in lungs
9. Heartworm
   
10. Obstruction of air passage

So, it will be good to take your dog to vet because different treatment options are recommended for different causes.