caffeine dehydrates, so it doesn't matter if there is water in coke, you still need to replace the fluid by drinking water or a non-caffeinated drink. the reason you drank less water before is not because you were getting what you needed, but because your body got used to your habits and compensated. Now it has realised that you are drinking water and so has re-adjusted. I bet you anything that if you stayed off the coke for a couple of weeks, on your first day back to your old habits you would develop a headache.
btw, i'm assuming that all you drank before is diet coke. Did you not drink any water before?