From simple 2 seriou. There are many questions that would need to be asked to give idea what is causing this. It could be neck muscle tension, sinusitis (see my answer relating to referred pain), migraine, tension ha...If associated with sore throat and fever, meningitis. If worst pain ever got there, subarachnoid hemorrhage, or brain aneurysm. Anything suddenly new, or severe, in that area has to be evaluated urgently.