I've been suckered into that too on a route I didn't know. My guess would be just low glycogen. You didn't bonk because you were eating, but your power and I bet HR dropped off. Did you modify your diet when you cut your mileage back? It is a good time to take a little rest. I just took a week completely off. My standing HR this morning was 59. I'll do a long high cadence drill on the rollers today. Gotta stimulate 'em after this rest.