This is one of two big things.
For me, this would happen to me when I got my self into the redzone. For me, the red zone is about 80-95% max heart rate, and it feels like I can't breath and seems to match the description of Exercise Induced Asthma (EIA). ever since I've adopted a HRM and Cadence to judge exertion levels, I've been able to avoid this issue.
Do get checked out by a doc just to be sure.
flyingcadet