Belly shook

Hey ladies. So today I had a little scare. I am 36 weeks and was sitting down and all of a sudden my belly shook. Shook violently for a good 5-6 seconds. Then did it again. Then I freaked and called my doc. Went in, did NST and everything was fine. But they don’t know what the shake was. Baby is still head down. anyone ever have this????