Can a baby give himself SBS (Shaken Baby Syndrome)?

My 5.5-month-old baby boy reached all the baby development milestones for his age. At 1m old he could already hold up his head for quite a while when he's lying on his tummy. He also turned it from side to side (I'd say he can do it briskly, especially in the early morning when he wants to bring himself back to the sleep again).In the following 3 months, he had attempt to roll over, and loved doing mini push up. At 4m1w he could roll over easily. Now he's trying to crawl and sit (still loving doing mini push-up but with arms straight). He however, never wants to "get down" from the tummy position (because he has a whole better perspective from there). After some time (four, five minutes) his neck gets tired and his head just simply "free falls" just as a branch with something heavy at the end point falling off the tree, to the point that his face almost bounce on the mat. But he resists that fall, like having an "emergency break" when his face is almost all the way down, and try to lift his head right away.I don't know how it's different from all the videos that told me blah blah that there's a space between the brain and the skull, and any sudden movement may cause the brain to smash against the skull. In my eyes dropping the head so fast and raise it back right away can also stop the skull and then make the brain push against it.Can this behavior, and the head turning left and right during his sleep cause SBS? I know I sound a little paranoid asking this question but I still feel unease watching my son "nodding" his head so fast while playing. May be I've read to much and take things too seriously. via /r/Parenting http://ift.tt/2h7p6PP