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 patient's initial presentation. The progression of pain and lack of relief from over-the-counter medications could indicate a more severe or prolonged episode.

Migraine: Although the patient does not report nausea, migraines can occur without nausea, and the severe, worsening headache could be consistent with a migraine, especially if the patient has a history of migraines.
Cervicogenic Headache: Given the initial localization to the posterior aspect of the head, a cervicogenic headache, which originates from the cervical spine, is a possibility. The pain's spread to other areas of the head could indicate referred pain from cervical structures.
Chronic Daily Headache: If the headache has been persistent for more than 15 days per month, it could be classified as a chronic daily headache, which includes various subtypes such as chronic tension-type headache and chronic migraine.

Subarachnoid Hemorrhage (SAH): Although less likely given the gradual onset, SAH can present with a severe headache and is a medical emergency. The absence of nausea and the progressive nature do not rule out SAH entirely.
Meningitis: Infection of the meninges can cause severe headache, and while fever and neck stiffness are common, they are not universal. The lack of nausea and recent medication changes does not exclude this diagnosis.
Temporal Arteritis (Giant Cell Arteritis): This condition, more common in older adults but possible in this age group, can cause severe, persistent headaches and is a medical emergency due to the risk of vision loss.
Brain Tumor: A space-occupying lesion could cause increased intracranial pressure, leading to headache, especially if worse in the morning or with coughing/sneezing. The gradual onset and lack of other symptoms (e.g., focal neurological deficits) make this less likely but not impossible.

Trigeminal Neuralgia: Characterized by intense pain along a branch of the trigeminal nerve, this condition is less likely given the description of the headache but could be considered if the pain has a specific, shock-like quality.
Idiopathic Intracranial Hypertension (IIH): Also known as pseudotumor cerebri, this condition can cause headache due to increased intracranial pressure without a mass lesion. It is often associated with obesity and can present with visual disturbances.
Headache due to Vasculitis: Various forms of vasculitis (e.g., primary central nervous system vasculitis) can cause headache, though these are rare and typically accompanied by other systemic symptoms.

The workup should be guided by the differential diagnosis and may include:

Complete Blood Count (CBC) and Erythrocyte Sedimentation Rate (ESR) to evaluate for infection or inflammation.
Imaging: A non-contrast CT scan of the head can quickly evaluate for hemorrhage, mass lesions, or other structural abnormalities. MRI of the head might be more sensitive for certain conditions like brain tumors or vasculitis.
Lumbar Puncture (LP) if there is suspicion of meningitis, subarachnoid hemorrhage (after a negative CT), or idiopathic intracranial hypertension.
Temporal Artery Biopsy if giant cell arteritis is suspected, especially in older patients.
Referral to a Neurologist for further evaluation and management, especially if the diagnosis is unclear or if specialized tests (e.g., electromyography for trigeminal neuralgia) are needed.