Strange Headaches

For the past couple years, I randomly get headaches that are usually on the left side of my head, towards the front. They come on suddenly and without warning. It is a throbbing pain, but if I sit down or lay down, they go away, but as soon as I move my head again, they are back. I have been to the doctor, and nothing has really come of it. My mom doesn't think they are a big deal, but I have had them a lot recently, and I have been missing a lot of school because of it. What can I do? Why do I only have the headache when I stand or move my head???