5 week old suddenly crying non stop

I have a 5 week old almost 6 and he all of sudden has started crying non stop for past 2 days. He will wake up 20 minutes (almost on the dot) every nap extremely unpleasant. Last night was the first night he didn't sleep his usual 3-4 hour blocks, instead it was 2 hours and woke up crying where before he would just wake up and make little noises. Anyone have experience with this? 

Just for background we've been following a routine which worked quite well until 2 days ago. Sleep, feed, diaper, wake time and then nap, repeat every 2 hours except for night. At night we do a feed, bath, story and then sleep. Wakes up 3-4 hours later and feeds usually twice before next morning.