Not sure, that's strange... they say digestion diverts some of the blood flow... but that is very dramatic.
I'd attribute it to normal cycles the body goes through... you can be on a high for so long and then your body grabs its recovery without warning, and just doesn't cooperate.
If you have upped the intensity in general, it may have finally showed up this one ride...
I was training for a halfironman and 4 weeks from race day I just went to blah energy levels for 2 weeks!!! I couldn't get enough sleep, and I couldn't get a good enough workout in... after 2 weeks I was good again, and hopped back out there ready for race day, but overtraining hits your body first, and you realize what happened later...
Then again, everyone has a bad day every once in a while!