There are many causes of shortness of breath, the most common one is probably obesity and lack of fitness. Furthermore uncontrolled lung diseases like asthma or chronic bronchitis can cause breathlessness. Breathlessness is unlikely to be serious if it does not occur during exercise. Occasionally an irregular heart beat, that only occurs intermittently, can cause breathlessness. It is always worth checking your pulse and if it is not regular see your doctor.
There are many causes for chronic breathlessness that need medical attention and if you are not 100% sure please see your GP and get assessed.