Can Anyone Please Explain?

Can anyone please explain why my two-year-old son has suddenly forgotten how to fall asleep?  Three times (so far!) in the past week, he has laid in his bed tossing and turning for one-to-two hours!  He doesn't whine.  He doesn't call out.  He doesn't cry.  He doesn't get out of bed.  He just lays there, rolling back and forth, switching positions.  He'll pull the covers up, then kick them off.  He lays at the head of the bed, then the foot.  He sits up, then flops back over in half and starts rolling again.  I've tried rocking him, but he tries to stretch out and there's just not enough room.  I've tried sitting next to him on the floor patting his back. I've tried both while humming his favorite lullabies and in complete silence.  I watch on the video monitor as, even when he's relatively still, his feet continue moving, thus indicating he's still not sleeping.

He has NEVER had this problem before.  I mean, this is the kid who slept through the night at 5 weeks!  It used to be that twenty minutes was a long time for him to stay awake.  I know he's tired.  And I know he's frustrated with not sleeping.  When I carry him upstairs, he'll drop his head onto my shoulder and rub his eyes.  I don't have the foggiest idea what's going on in his brain, since he can't tell me.  And I don't know how to fix it!

And while you're at it, can you please explain why, when he woke up from his nap today, he cried continuously and inexplicably for 15 minutes?  I offered him milk.  He cried.  I offered him a graham cracker.  He cried.  I tried to hold and comfort him.  He kicked me, pushed me away, and cried.  After leaving him alone, he decided he was done crying, came up to me all smiles, and was pleasant the rest of the day.  ARGH!