Do you use a heart rate monitor when riding? Could you be taking any meds that affect your performance and abilities?
I'm nowhere near a competitive rider, but earlier this summer we went mountain biking on St. Maartin. About half way up one steep hill, I felt that while I had plenty of "muscle" I had no "wind." I couldn't get my breath, no matter how hard I huffed and puffed. Finally, I had to get off the bike and walk to the peak. Once there, I stopped, caught my breath, and finished the ride in fine shape. When I got back, I talked with my electrophysiologist, complaining that my pacemaker wasn't keeping up with me. She said that it wasn't my pacemaker, as I was naturally pacing most of the time. She laid the blame on my beta blocker, which limited my heart rate to about 140 or so - which apparently wasn't enough to get up the hill at the pace I wanted.
So, long story short, you may be taking meds that have little to no noticeable effect in everyday activities, but which may be limiters like a brick wall under heavy load.