If you've ever been so thirsty that no amount of water seems to slake that thirst, chances are there is something going on inside your body that needs to be evaluated by a health-care provider. The headache might be caused by the unquenchable thirst and should be added to any list of symptoms you're giving. 

Considerations
Drinking lots of water is usually healthy. However, the urge to drink too much may be the result of a physical or emotional disease. Excessive thirst may be a symptom of high blood sugar (hyperglycemia). It can be an important clue in detecting diabetes. Excessive thirst is a fairly common symptom. It is often the reaction to fluid loss during exercise, or to eating salty foods.

Common Causes
-- A recent salty or spicy meal
-- Bleeding enough to cause a significant decrease in blood volume
-- Alcoholic Liver Disease
-- Diabetes
-- Diabetes Insipidus
-- Drugs such as anticholinergics, demeclocycline, diuretics, phenothiazines
-- Excessive loss of water and salt (possibly due to not drinking enough water, profuse sweating, Diarrhea, or vomiting)

When to Contact a Medical Professional
Call your health care provider if:
-- Excessive thirst is ongoing and unexplained.
-- Thirst is accompanied by other unexplained symptoms, such as blurry vision and fatigue.
-- You are passing more than 5 quarts of urine per day.