Although i find it very unlikely to concuss yourself whilst asleep with no memory of it, i dont think its completly impossible. I had something very similar when i was a kid. I whiplashed myself during my sleep with no recolection of it happing. Doctor did confirm it was a proper whiplash. But back then i had something called pavor nocturnus, which is a condition where you have violent panic attacks every night in your sleep with complete amnesia.