Ah the blast cry. We had those, they're really lovely because it takes you, the parent, forever to go back to sleep after the shocking wake up. We didn't get them until DS1 was around 10 months but they lasted on and off for almost a month and then they just stopped. It was a very bizarre thing, we have no idea why they occurred but we learned to wait for a minute before going in. They never lasted longer than 20 secs max, and like you said, he appeared to be sleeping the whole time. VERY stressful but hopefully short-lived.