Shortness of breath has many different causes.
Obstruction of the air passages of the nose, mouth, or throat may lead to difficulty breathing.
Heart disease can cause breathlessness if the heart is unable to pump enough blood to supply oxygen to the body.
If the brain, muscles, or other body organs do not receive enough oxygen, a sense of breathlessness may occur.
Sometimes emotional distress, such as anxiety, can lead to difficulty breathing.
Specific causes include the following:
Coronary artery disease
Heart Attack (myocardial infarction)
Interstitial lung disease
Rapid ascent to high altitudes, with less oxygen in the air
inhalation of a foreign object
Allergies (such as to mold, dander, or pollen)
Deconditioning (lack of exercise)
Compression of the chest wall
As you can see there are many possible reasons and your Doctor will need to evaluate you further.