Kitten Shiver/Tremors while falling asleep

My 14 week old kitten has recently begun to shiver or have tremors when
he lays down and begins to fall asleep. His entire boddy shakes
slightly as though he is shivering from cold. I have attempted to warm
him up by covering him with a blanket but this does not stop the
shivering.

He had his second booster vaccination last week, could this be a side
effect? He does not have this symptom when he is awake and appears to
be in good health (good appetite, plays well, purrs, drinks plenty of
water, and has no other noticeable behavioural change since the
shivering began).

Does anyone have any ideas why he's shivering? Could it be just a
natural phase of sleep or something more serious?

Help!