Although there is a condition called hyponatremia, generally you will drink to your thirst level. Hyponatremia is when you sweat so much that you lose sodium and electrolytes and are only replacing water. This is a concern when exercising heavily outdoors for long periods at a time (running a marathon in the heat, for example).
You don't mention how much water you are drinking. A great way to tell if you are getting enough water, is to check your urine output. If it's the color of apple juice, you could drink more. If it's clear or very light, you are well hydrated already. It should look like a glass of water with lemons squeezed into it (slightly yellow, but not dark).
Here is a great article that might help: www.sparkpeople.com/resource/fitness_artic