What Causes Frequent Gasping In Infants?

My son is 5, and over the past 5 days he s begun gasping for breath only while sitting. He s able to run and lay normally and I don t notice this breathing while he s sleeping. But I am concerned that when he sits still while playing games, coloring, watching movies, etc. he repetitively takes deep breaths, saying he has to . I took him to a pediatrician last week to rule out asthma, which he did. 4 days later it seems to be getting more pronounced. What could it be? He has environmental allergies, but generally very healthy: and is about 39 lbs.