Most likely cause of shortness of breath if lung function good chest X-ray clear and no asthma or allergies?

More details needed. Do you have SOB at rest or only with or after exercise? Do you have trouble getting air in or pushing air out? Do you have trouble talking whenever you have such an attack? Do you have any pounding in your chest? Do you have anemia? Providing such info will likely get you a better answer.since there are so many causes of SOB.
Shortness of breath. Shortness of breath in healthy people could be sometimes related to unknown arrhythmia or just simple anxiety.