Shaking/vibrating feeling from baby?

can't say exactly when but all of a sudden out of nowhere I'll feel LO sort of shaking or vibrating almost like a violent flutter! This lasts for a couple of seconds but it always creeps me out when it happens! It's mainly if I'm lying still that I'll notice it though. It's almost like he's having some sort of seizure in there or something.Scans have all been fine etc and I know its not hiccups bcos hiccups are rhythmic. Maybe it's stretching or something.. I don't know!?

Anyone else had this??

****UPDATE*****

WOW! thanks for all the replies, wasn't expecting much...I have felt it since I wS 37 weeks (am now 37w 5d) only felt it once a day since then. Some make a lot of sence, the stretching or I thought it was like goosebumps you get when you pee. Idk but as long as my baby is healthy and doing good that's what matters, it was a little concerning at first that's all.