I think all of your guesses about the cause are likely. Another possibility, is that maybe you were sick, but not enough to notice in normal day to day activities. I've had that happen. I got dropped like a bad habit on a group ride one time very quickly (quicker that usual that is :-). I pretty much limped home. I just didn't have it in me to keep up but couldn't figure out why. I was sick a day later with a low grade fever. Also, if I've had a few days of little activity and maybe eating not as good as I should I feel it on the bike big time. I find that if I have a ride planned, I need to pay closer attention to what I'm eating/drinking at least a day in advance or it will negatively impact how I feel on the ride. Of course I should pay better attention to that all the time, but that is for a different thread.... Everyone is different. You just have to pay attention to what your body is telling you. Cycling has helped me learn a lot about how my body works best.