What the hell? Baby sleeping.....?

My 3 month old coos and babbles in his sleep and occasionally screeches but just now I heard him fussing (loud enough to be heard in the kitchen with the sink running), so I went in to check him. He was completely asleep. I turned to walk away and he started fussing again and writhing around but he sounded funny so I looked at his face. Completely asleep! He made some really funny noises and was squirming all ove rhte place but never woke up. I left the room and heard him again, went back in, he was asleep. What hte hell is he doing? I feel liek I shoudl wake him up if he's having a bad dream but what could a 3 month ol possibly be dreaming about that makes him fuss? He had been pretty fussy earlier tonight if that has anythign to do with it. Do all babies do this?