What is wrong with my breathing?

sometimes i would have these episodes when i can't breath right. its like i am not satisfied when i take a huge deep breath. its like something is blocking my airway. it can't be phlegm, i know that for sure. i know i am asthmatic so maybe that could be the problem. i was born prematurely so i guess that could have been a factor developing my lungs. but sometimes i would have these times where i could not inhale because of this sharp pain in my side. it has happened before. many many times. does anyone know what this could be? sometimes when i discomforted because i'm sitting in a weird position i tend to feel a shortness of breath too. when i exercise or run i'm completely fine. any doctors out there? lol