Shortness of breath for 2 months, clear echo a year ago, clear chest xray/ekg/d dimer in december. could this be lung cancer or asthma/allergies?

Could be asthma: These symptoms are nonspecific . Depending on the type of cough, productive or nonproductive , will help determine the cause. Chest CT should be considered since other tests are normal. Best to talk to your doctor to get to the bottom of the cough

Rare vs common: Lung cancer would be exceptionally rare at your age (31). Asthma is very common. In medicine, there is an old adage, "Common things happen commonly. If you hear hoof-beats, don't look for zebras." Pulmonary function testing with methacholine challenge if necessary is indicated.