Serious health question

[a completely inoffensive name]

If your urine is clear, water isn't the problem. If you are going through very rigorous exercises in high heat, the issue might be that you are lacking essential electrolytes. When you sweat, you lose ionic forms of potassium, sodium, magnesium, calcium etc... Drinking pure water creates a diluted system in your body is which bad because your body needs those ions for sending electrical signals.

These electrical signals are the basis for the control of your muscles, and if your muscles are in pain/acting up it may be a sign that your electrolytes are not at their proper proportions in your body.

http://en.wikipedia.org/wiki/Hyponatremia

Symptoms 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.