Tension Headache: This is the most common type of headache and can be triggered by various factors including stress, muscle tension, and even the temperature change from a shower. The description of the headache being behind the head and aching, worsening after a head shower, fits with the characteristics of a tension headache, which often involves a band-like pressure around the head.

Cervicogenic Headache: This type of headache is referred from the cervical spine and can be exacerbated by movements or positions that put strain on the neck, such as the act of showering. The pain is typically on one side but can be bilateral and is often described as a dull ache.
Sinus Headache: Although less likely given the specific location behind the head, sinus headaches can occur due to sinus pressure and can worsen with changes in temperature or humidity, such as during a shower. However, they are usually accompanied by other sinus symptoms like congestion or facial pain.

Subarachnoid Hemorrhage: Although extremely unlikely given the description, any sudden, severe headache (often described as "the worst headache of my life") warrants consideration of a subarachnoid hemorrhage, especially if accompanied by other symptoms like nausea, vomiting, or loss of consciousness. The sudden change in intracranial pressure during a shower could theoretically exacerbate symptoms.
Meningitis: Infection of the meninges can cause headache, fever, and stiff neck. While the headache pattern doesn't specifically suggest meningitis, any headache accompanied by fever, confusion, or stiff neck should prompt an urgent evaluation.

Occipital Neuralgia: This involves inflammation of the occipital nerves and can cause severe, sharp, or stabbing pain in the back of the head. It could potentially be exacerbated by the water or temperature change from a shower.
Temporal Arteritis: This condition, more common in older adults, involves inflammation of the blood vessels in the head and can cause headaches, often accompanied by jaw claudication or visual disturbances. While rare and less likely given the specific scenario, it's a diagnosis that should be considered in the appropriate demographic.