What the hell?

K, I was in my 2:30 lecture, half-dozing off near the end, because I had been up for ~18 hrs, with a small nap at lunch.

Anyways, I was sleeping with my arm propping up my head, and I remember my head kind of bobbing back and forth.  All of the sudden, my head slipped backwards, and as it slipped my body's autonomous reflexes caused it to snap back right.  I don't think I got whiplash or anything (nothing is sore), but as this event happened, it felt like some surge of energy or adrenaline shot up through my neck and it completely woke me up.

It's  about 1.5 hrs later now and I feel less tired than I did before my afternoon class even started, and I don't even  want to go to sleep now.

What the hell just happened?

Probably the nap refreshed you and that's it. The head snapping back is just normal reflex I would assume. Either that or you had a ministroke and should go to the doctor now.