Tension Headache: The patient's symptoms of a unilateral headache with a significant decrease in blood pressure could be indicative of a tension headache, which is a common condition often triggered by stress, fatigue, or other factors. The decrease in blood pressure might not be directly related to the headache but could be a coincidental finding or related to other factors such as relaxation or decreased stress after being seen.

Migraine: Migraines can cause severe, unilateral headaches and are often accompanied by other symptoms such as nausea, vomiting, and sensitivity to light and sound. The change in blood pressure could be related to the migraine itself or to the body's response to pain.
Cluster Headache: Although less common, cluster headaches are extremely painful, occur in clusters or cycles, and are typically felt on one side of the head. They can be accompanied by autonomic symptoms like nasal congestion or eye watering on the same side as the headache.
Hypertension-related Headache: Given the initial high blood pressure reading, it's possible that the headache could be related to hypertension, especially if the patient has a history of uncontrolled high blood pressure. However, the subsequent decrease in blood pressure might argue against this as the primary cause.

Subarachnoid Hemorrhage: Although unlikely given the information, a subarachnoid hemorrhage (bleeding into the space surrounding the brain) can cause a sudden, severe headache and can be associated with changes in blood pressure. It's a medical emergency that requires immediate attention.
Temporal Arteritis: This condition involves inflammation of the blood vessels in the head and can cause severe headaches, often on one side. It's more common in older adults and can lead to serious complications like blindness if not treated promptly.
Pheochromocytoma: A rare tumor of the adrenal gland that can cause episodes of high blood pressure, which might be accompanied by headaches. The significant initial blood pressure reading followed by a decrease could potentially be related to the episodic nature of this condition.

Trigeminal Neuralgia: A chronic pain condition that affects the trigeminal nerve, which carries sensation from your face to your brain. It can cause intense, shock-like or stabbing pain in parts of your face, which might be mistaken for a headache.
Occipital Neuralgia: Involves inflammation of the occipital nerves, which can cause severe pain on one side of the head, often starting from the base of the skull and moving up. It's less common and might be considered if other diagnoses are ruled out.