Thwarted

My son naps for 45 minutes at a time, you can usually set a clock by it give or take a few seconds. So I need to go to the library and pick up a book I ordered, it closes at 5 p.m. At 1:45 he went down for a nap, he woke up less than 30 minutes later screaming (not in his character) so I finally managed to settle him down again by 2:30 p.m. It is now after 4:30 and he is still asleep. Why on days that I count on him being predictable does he throw me for a loop?