I'll have to second (er... fifth) the asthma call. While it sounds mild, definitely talk with a doctor. I (almost) never get attacks while training unless I'm doing a long TT training ride where I'm working at peak levels for a long time. I still have an inhaler, though, just in case. Turned out to be a smart move one time: I'd been done with my workout for 30 or 40 minutes and was having mild breathing problems, but just figured I'd wait for them to go away. In the course of about one minute, they very suddenly got drastically worse, and I could barely breath at all. Imagine trying to breath through a coffee stirrer; that's how bad it was. Luckily I had my inhaler nearby; I can't imagine what would've happened if I hadn't.