Salam,
There are many causes of being short of breathing.
Among them:
1) Diseases affecting the lung it self , like Bronchial Asthma, chronic bronchitis, interstitial lung disease, pulmonary hypertension, ...etc.
2) Diseases affecting the chest wall, like scoliosis, kyphosis, rib diseases, muscular diseases, ...etc.
3) Diseases affecting the pleural membrane (around the lung and overlying the rib cage from inside), like pleural effusion, pleurisy, pleural tumors,..etc.
4) Diseases affecting the muscles, like muscular inflammation (myositis), muscular dystrophy, diaphragmatic paresis,..etc.
5) Diseases affecting heart, like heart failure.
6) Diseases affecting the esophagus, like heat burn (relux), esophagus spasm, ..etc.
7) Compression on the wind pipe by abnormal vessels, thyroid gland, tumors or others.
7) Some times it is psychological stress.

This is very short over view about the causes of breathing difficulty, and as you see it is a wide group of diseases, and only the specialist physician will be able to differentiate these causes and do the needed investigations.