Asthma is a possibility. When you ride, do you have problems with wheezing or coughing? Do you feel like you can't inhale fully, like your lungs won't expand all the way?

A pulmonary function test would show if you have impaired lung function, which could be caused by asthma. They're quite effective -- the last one I took showed that my asthma was getting worse before I started to feel worsening symptoms.