It wasn't until I backtracked that I found the culprit. The problem was that I hadn't eaten enough today. Breakfast was the usual, so that got me through the early training sessions, but lunch was seriously lacking. I can only blame myself as my mind was set on using the precious couple hours I had free today to study for my next certification. In the whirlwind of excitement, I downed some veggies and chicken and a touch of pasta before breaking out the study material.
After finishing today's study, I began my routine. What was supposed to be a solid 60 minutes ended up clocking in around 40. While I had the right amount of protein for my weight and enough veggies to kill a small horse, my complex carb intake was definitely lacking. As such, my glucose levels were low and I had started to rip into my glycogen stores.
Lesson of the day (mostly to remind myself): EAT! If you're burning fuel, you first need fuel to burn, dummy.