MY SON IS 5 MONTHS OLD AND I BREASTFEED. NOW WHEN HE IS EATING AND DRIFTING INTO SLEEP HIS HEAD AND THE REST OF HIS BODY STARTS SHAKING. NOW HE DOES IT SOMETIMES WHEN HE IS ASLEEP TOO. I CAN STOP THE SHAKING BY GRABBING HIS LEG FIRMLY, BUT AS SOON AS I LET GO HIS STARTS SHAKING. NOW IS THIS JUST LIKE MUSCLE SPASMS OR SOMETHING OR IS IT MORE SERIOUS?