My bet is on EIA, or exercise induced asthma. I have it, as do a large part of the population. For me, weather affects my tolerance of it as well. When I ride on really hot days, or really brisk cool days it seems worse. You need to go to your doctor right away, though, so you don't have a problem again and no med to help. Usually, they will give you an albuteral inhaler to use about 20 minutes before exercise-for me makes all the difference in the world on breathing AND performance. But you should take it with you on long rides in case you have an unforseen problem. Do you cough a lot with exercise, or after laughing hard? Can't quite seem to clear your lungs when riding??