HEADACHE after acute trauma is common and transient, whereas chronic headache is rare and persistent. According to the International Headache Society, posttraumatic headache (PTH) becomes chronic when it lasts more than 8 weeks. Saper argues that PTH is a neurobehavioral disorder, involving genetics, sex, neuroanatomy, and physics. He refers to a number of possible mechanisms, including injury to dorsal raphe nuclei of the brainstem, thought to be the migraine "generator." He cites studies that show that psychological symptoms follow the headache and not the other way around, and suggests that once a neurophysiological disturbance exists, a number of factors, including stress, can modulate it and result in pain.
Hachinski V. Posttraumatic Headache. Arch Neurol. 2000;57(12):1780. doi:10.1001/archneur.57.12.1780