Because, it turns out, if you do two hard workouts within 12 hours without having set foot in a gym for six months, you will feel paralytically sore the following day. I actually woke up in the middle of the night and realized that I was so sore I couldn't actually turn over. Wrestling my pillow back into place was an agony, and getting out of bed was an Herculean task that was nearly all I was worth this morning.
Thank the powers that be for copious amounts of ibuprofen and relafen. In this manner, I was able to drag myself to work using my eyebrows (which didn't get a work out, and thus were only sympathetically sore, not actually sore) - and by the time I got here, I was just able to flop my velociraptor-like arms up onto the keyboard rest, where they shall remain for the rest of the day.