I had these headaches recently for many days. I also noticed soreness in my legs (but not from exercise, I wasn't exercising). After some thought I cut back on my water consumption drastically, only drinking when thirsty. I was afraid to do so because in the past, dehydration would trigger migraine headaches (when I was eating a SAD). But cutting back on water got rid of the headaches and the soreness is much diminished, I also have a bit more energy at the end of the day! I think I may have been suffering from Hyponatremia.
From:
http://en.wikipedia.org/wiki/Sodium_deficiency#CausesDiet. A low-sodium, high-water diet can disturb the proper balance between sodium and fluids in the blood. Excessive intake of diuretics, including beer, can have the same effect.
And:
http://en.wikipedia.org/wiki/Sodium_deficiency#SymptomsSymptoms of hyponatremia include nausea and vomiting, headache, confusion, lethargy, fatigue, appetite loss, restlessness and irritability, muscle weakness, spasms, or cramps, seizures, and decreased consciousness or coma.
So the lesson here is to not worry about water, just drink it when you are thirsty!