Baby slamming own head against mattress?

Lo

Any one else's baby do this?

When laying on his back he turns his head to one side and basically slams his head into the mattress or pack N play over and over.

Asked Dr about this multiple times and she didn't seem concerned (even showed her video) but sometimes it seems SO hard!

Worried he's hurting himself! He does it quite frequently... when upset/crying/fussing and when waking up at the night for food/change.

Anyone else??