If you're drinking that much water in a day and working out, you are probably flushing electrolytes out of your body. This will result in the symptoms you describe. You need to be careful w/ that much water, I had a good friend who spent 4 days in ICU because he flushed all the electrolytes out of his system.
Try adding a little more salt to your diet and a little gatorade. Also, look around and see if you can find coconut water. I get it at my local grocery store, but if you can't find it there, look at a health food store (if you can stand doing business with dirty hippy retreads). NOTHING replenishes electrolytes better than coconut water. I'd bet dollars to donuts that at least part of yout problem is electrolyte deficiency.