How are your lungs?

Being able to do long distance riding at sub-threshold intensity (e.g. threshold being how hard can you ride non-stop for an hour) is not the same as efforts at 150-200% of threshold which you are doing when climbing.

If you never ever do any riding at 200% for 60-120 seconds except on these rides then they will hurt.

I think this is probably more likely than effortt induced asthma.