It could be quite a few things. My guess is that it is probably just a pinched nerve (w/ occiptal nerve involvement). When you go to the doctor, s/he will probably prescribe a course of steroids (maybe some muscle relaxants) and a few sessions of physical therapy.
If this doesn't solve it my guess is the next move would be to try a couple migraine meds (maybe a tricyclic and an AED). If this this didn't solve it they would probably diagnose as occipital neuralgia or as a secondary headache (other treatments). This is the typical diagnostic progression. In all honesty, the steroids (muscle relaxants) and PT will probably clear it up.
Your doctor may want a head and neck CT, just to be safe. It wouldn't be the worst idea.
BTW, did you get into a car accident or hit your head within the past few months (yes... months)? If so, definitely mention it to your doctor.
Best of luck!
Post Edited (EvilFluorescents) : 3/4/2011 12:43:37 PM (GMT-7)