Tension Headache: This is the most common type of headache and is often described as a band or a squeezing sensation around the forehead, back of the neck, or both, which matches the posterior location described. The persistent nature over 5 days could indicate a chronic tension headache.

Cervicogenic Headache: Given the posterior location of the headache, it could be referred pain from the cervical spine. This type of headache is often associated with neck movement or pressure on specific areas of the neck.
Migraine: Although migraines are typically unilateral and can be accompanied by other symptoms like nausea, vomiting, and sensitivity to light and sound, some individuals may experience migraines that are bilateral or have a posterior component.
Sinusitis: Inflammation of the sinuses can cause headaches, although these are more commonly frontal. However, given the variability in sinus anatomy and the potential for referred pain, a posterior headache could be considered, especially if accompanied by other symptoms like facial pressure or nasal congestion.

Subarachnoid Hemorrhage (SAH): Although less likely given the duration and lack of sudden onset, a SAH can present with a severe headache and is a medical emergency. The classic description is a "thunderclap headache," but not all SAHs present this way.
Meningitis: Infection of the meninges can cause headache, fever, and stiff neck. While the presentation is usually more acute, some cases can have a more gradual onset.
Temporal Arteritis: This condition, also known as giant cell arteritis, can cause headaches, typically in older adults, but it's essential to consider in any new onset headache, especially if accompanied by visual symptoms or jaw claudication.

Occipital Neuralgia: Involves the nerves that run from the base of the skull to the scalp and can cause severe, sharp, or stabbing pain in the posterior part of the head.
Chiari Malformation: A structural defect in the cerebellum that controls balance can cause headaches, often worsened by coughing, sneezing, or straining, and can be associated with posterior headache due to the anatomical location of the malformation.
Pseudotumor Cerebri (Idiopathic Intracranial Hypertension): Characterized by increased intracranial pressure without a detectable cause, leading to headaches that can be posterior, along with other symptoms like vision changes and tinnitus.