How do i know if my little dog has a breathing problem because he is short of breath often?

I have a little dog about 6 months old, a chorkie (chihuahua/yorkie), and he gets short of breath sometimes and starts breathing really fast to catch his breath. Do you think he has a lung or breathing problem and is it common in most small dogs?

I have a chihuahua that is 6 years old now, and sometimes when she gets overly excited she will make grunting or breathing noises for 5 to 10 seconds . We pick her up and gently rub her throat and calm her down and it goes away.

I took her to the vet, and she said it is common in some small dogs. Mine has had no other effects, and is otherwise very healthy and playful. But is good to get him tested to see if is common, or anything serious.