A bonk is when you burn enough energy to deplete your glycogen stores without consuming enough carbohydrates to replenish them. This usually takes about 2 hours, but can happen faster if the exercise is more intense or if your glycogen stores were depleted at the outset. Your blood glucose drops and you feel symptoms of hypoglycemia, you feel irritable, sad, shaky, sweaty or you just fall over. Bonking really badly is called 'cockroaching' for the way the insect behaves flipped on its back.

Sounds like you might have needed a rest day, or that you maybe didn't eat enough (esp carbs) after your last ride. You also may have started off a bit dehydrated, or any number of things. But a bonk is something different. As they say, you'll know it when it happens.