What's causing my headache i've had this pain in the back of my head for six days

It : It could be a tension headache, especially if the muscles in your neck or upper back are tight.

Headaches : Headaches fall into two different categories. The first category is the primary headaches. These are headaches like migraine, cluster headache, and tension headache. They happen "just because" and are not caused by a serious underlying problem. People with primary headaches have a pattern of similar headaches that go back over years. The second group is the secondary or symptomatic headaches. These are the headaches that are symptoms of something wrong. People with these kind of headaches notice that they are new, or different from any headaches they have had before. Secondary headaches can be symptoms of some very serious problems, including infections, aneurysms, arterial dissections, brain tumors, increased pressure inside the skull, inflammatory diseases and other problems. Any headache that sounds like a secondary headache should be evaluated by a physician.