This has happened to me a couple of times. My SO and I went to a sponsored group of rides over a 4-day period this past week. The weather was hot, but not humid. I'm very healthy and in pretty good shape & didn't feel that this particular ride was any harder than some of the rides we do every week. At about 3 miles out from the end of this 50-mile ride I started having breathing issues. My lungs felt restricted, and I was unable to get air. Naturally, the more I had trouble, the worse I got. As we didn't have a paper bag to breathe into (it was kind of like hyperventilating, but different), I (stopped riding, of course) put my hands over my mouth, to try to slow the gasping, and after a few minutes could breathe again. I was a bit shaken, but ok to ride. We finished the ride, and at the end, when I got off my bike, it started again.
I don't think I'm of the age range to start with asthma, but it felt like that...

Any ideas? Anyone ever had that happen before? Do you think it's something the doc should check out?