Diabetes Insipidus: This condition is characterized by the inability to regulate fluids in the body due to a problem with the antidiuretic hormone (ADH), leading to symptoms like frequent urination and excessive thirst. Headaches and neck pain could be related to dehydration or the underlying cause of the condition.

Diabetes Mellitus: Although primarily known for high blood sugar, diabetes mellitus can also cause frequent urination (polyuria) and increased thirst (polydipsia) due to the body's inability to use glucose properly, leading to its excretion in urine. Headaches and neck pain could be secondary to dehydration or poor glucose control.
Overactive Bladder: This condition could explain the frequent urination but might not fully account for the excessive thirst and headaches unless there's an underlying issue like a urinary tract infection causing discomfort and leading to increased fluid intake.
Dehydration: Despite the constant drinking of water, dehydration could still be a factor if the patient is losing more fluids than they are taking in, possibly due to an underlying condition. Dehydration can cause headaches and potentially neck pain due to muscle strain.

Meningitis: Although less likely, meningitis is a serious and potentially life-threatening condition that can cause headaches, neck pain (due to meningismus), and possibly altered mental status. Frequent urination and thirst might not be primary symptoms but could occur due to systemic infection or inflammation.
Subarachnoid Hemorrhage: A severe headache (often described as "the worst headache of my life") and neck pain could be indicative of a subarachnoid hemorrhage, a life-threatening condition. While not directly related to urination and thirst, these symptoms could be part of a broader clinical picture including altered mental status or systemic responses to severe pain.
Pituitary Tumor: A tumor in the pituitary gland could affect hormone regulation, including ADH, leading to symptoms of diabetes insipidus. Additionally, headaches and neck pain could be due to the tumor's size and location, putting pressure on surrounding structures.

Nephrogenic Diabetes Insipidus: A rare condition where the kidneys are unable to respond to ADH, leading to an inability to concentrate urine and resulting in polyuria and polydipsia. Headaches and neck pain could be secondary to dehydration or an underlying genetic condition.
Psychogenic Polydipsia: A condition where excessive water drinking is driven by psychological factors rather than physiological needs. While it could explain polyuria and polydipsia, it might not directly account for headaches and neck pain unless these symptoms are also psychosomatically driven.