Why am I having regular headaches above the back of my neck?

Headaches. These headaches could be due to stress, poor posture, needing a new pillow, or a lack of restful sleep. Depending upon the intensity, frequency duration, and location, there could be more worrisome causes. Best place to start is to talk to your doctor about these symptoms so he/she can sort it out. Good luck!
Could be posture. Lying on a sofa or bed with your head elevated by a pillow can put undue stress on the neck. If there are other symptoms like tingling of your hands, pain that radiates to a shoulder, or weakness/atrophy of muscles consider a medical evaluation that includes cervical spine x-rays. Any history of trauma to your neck? What is your posture like at work - in general? What is the nature of your work?