Could be a pulled, strained, or irritated muscle from traps to occipital. Ice, massage and a visit to the chiropractor tomorrow are suggestions.
What you are describing just might be a symptom of temporal arteritis. Are you having other symptoms--like weakness or nausea?
By any chance have you experienced a head injury? Sometimes those side effects can occur right away or can come later on. For example, I had a concussion and the symptoms never showed until a week or so after the incident, I still will get severe pain every now and then and severe headaches. Another possible cause could be due to stress or like some of the above mentioned.
I have had nausea a little, feeling a little dizzy at times and my left check tingles.