My 21 month old son started about 8 weeks ago lying on his tummy with his hands crossed nere his nappy and stiffening up every night with out fail and it completely knocked him out its like a habit now he can't go to sleep with out doing it and most night he has to lye across my arm whilst he is stiffening he also sounds like he's holding his breath aswell an after 5 mins or so he's fast asleep what is it and should I be very worried ????