Why is my cat breathing different?

Just lately I've noticed my cat breathing differently. Her activity level has not changed. She is 3 months old and occasionally makes weird breath sounds. One time she hoped to the back of the couch and sounded the the air was pushed out of her lungs really fast. She didn't act weird and just went about her way. I have taken her for 2 pet exams for her normal check ups, and he says she is perfectly healthy kitten. I am just a concerned that it is something that he did not catch. Has anyone ever had a cat have this issue. Side not her diet and liquid intake is healthy.