You might have bad posture in your neck. In other words, your head is sitting too far forward.
I used to have a constant headache all the time and no idea what caused it. I don't know exactly the mechanism, but it causes pressure on your skull when your neck posture is bad, which causes headaches.
If you want to test it quickly, lay down on a bed with your shoulders lining up at the edge of the bed.
Let your head hang down over the edge of the bed and relax all of your neck muscles as much as possible (it helps if you put a hot wet towel around your neck first). Make it so that your head pushes back behind your shoulders as much as possible without causing pain.
Stay in this position for 5-10 minutes, and try to hold your head in the back position as you stand up.
If bad posture was causing your headache, you should feel some relief for a while as long as you don't go back to bad posture.