Wtf.
Know how I said my leg muscles always get tired first? Well today I went to the gym and felt a bit sluggish, got home, as I got ready to ride I was breathing really hard. Then I went to do 4x8 with 2' RBI. The first effort was ok, the next 2 were down around threshold with hard breathing and I just couldn't do the last one. My legs were fine. I was literally gasping for air. Weird. This never happens.