Similar thing happens to me every now and then, last time yesterday/today.
Sitting at the computer with itunes on shuffle, then I'm Broken (Pantera) comes up and i start to bang my head without thinking about it. And it goes through some sweet sweet songs, metallica, megadeth, AILD, In Flames, Bodom, lamb of god etc.
When i check my clock it's 3:30, that would be 4 hours ago from now. xD
And i got a test in spanish in one hour..