Have you tried massaging the back of your neck? A lot of headaches are caused by the tendons in the back of your neck getting stretched out. If you massage the back of your neck (massage in circles right on those tendons), the headache will go away temporarily... but at least you'll know what the cause is.

In order to make those sorts of headaches go away, I lay down and position my head on a pillow different from the way I normally do it. Sometimes positioning your head to the side can help, or letting your head "hang off" the back of your pillow. You basically want to make sure the tendons have a way to relax.

Hope that helps