Tension headache? This could be a tension headache. This usually presents as a band of pain round your head, like someone tightening a belt around your head. It can also present with tenderness on the back of your head. Definitely worth seeing your doc for this. Also should start a headache diary. Here's a helpful link for this. http://www.headaches.org/for_professionals/headache_diary.
Cervicogenic HA. The cervical (neck) spine is a frequent culprit that causes back-of-head pain. The occipital nerves that come out of the upper neck can become irritated by the joints or muscle of your neck and send pain up the back of your head and even across the top. Can try to relax the muscles with heat, muscle relaxation techniques, anti-inflammatories. Talk to your doc if above fails. Options galore!