You probably should have drank more during your first hour. Most people stand as a rule a bottle of water and 200 calories every hour. Even the starting one. Thats not always the cause of cramping though. Could be a lot of other things. edit: If you were only drinking water the second hour and so on because you felt you needed water you probably weren't drinking enough. By the time you feel like you need water is usually after the time your body actually does.