I can remember labour with my third baby much better because it was a drug free birth. And I remember at the end sort of "shutting down" in between contractions, just before pushing started. Eyes closed, not responding, not moving. I can vaguely remember DH saying "It looks like she's asleep" and the midwife saying "she might be". Not quite the same as what you experienced, I know, but it may have been something sort of similar, exaggerated by the pethidine shot you had. I think your body grabs whatever opportunity it has to rest as much as it can in between the awesome effort of pushing that baby out!