L.H.
My 3 month old does this often, especially when she's tired and trying not to fall asleep. She's been doing it for a month now. She whips her head back and forth real fast, like I wrote, mostly when she's tired. I'm not concerned because it doesn't seem to bother her and she's so active anyway. My nephew did it too and never grew out of it. Now he likes spinning around until he goes "splat" and hanging upside down on the monkey bars.
--L.