What Causes Difficulty In Breathing?

when my niece was 3 months old she started to have shortness of breath.....at one point she look like she did wanted to sneeze...then when she did it again i knew something was wrong so i took her to the docter..they ran all kind of test like newborn screening, ekg,blood work......an they didnt find nothing wrong.....she is 8months now an it is only happen if she cry too much thats when it is start to happen...i dont no if is shortness of breath or what?