I had this with both of mine. It was like a little vibration, and lasted for a couple of seconds.
When they came out, I saw what it was. When they would straighted out their leg, it would shake, vibrate, for a couple of seconds. It lasted for a couple of months, then went away.
I asked my ped about it, and he said he had seen it before, and not to worry. People would see it when they stretched, and would say, "what are they doing!?" It was just "their" thing!
It will be interesting to see if this is what your little baby is doing!!! Forgive me. I didn't read the other posts, so sorry if this is repeating what someone else has already said!!!!