Becky, you mention the 'headache stage' of dehydration.
Interestingly,headache is also one of the symptoms of certain electrolyte deficiencies:
http://www.healthatoz.com/healthatoz..._disorders.jsp
If you still have unpleasant symptoms and headache that drinking plenty of water won't cure, then it's possible that the headache can be coming from a lack of sufficient electrolytes or other nutrients as opposed to being due to dehydration (insufficient water).
And yes, when we sweat a lot and become dehydrated we also tend to lose electrolytes and various other nutrients.