Just did some digging on Google and came across this link...
http://www.pets.ca/forum/showthread.php?t=20889
Could it be low blood sugar? I know for certain that his sugar is low pretty much all the time when he won't eat. I doubt its epilepsy as I've never seen him have a seizure. What concerns me is that he went from a normal dog to a sick dog that shakes his head to a constipated dog with a Stevie Wonder head roll... all in the course of two weeks. I'm stumped. It just seems like he gets over one thing and something else comes along and attacks him.