If the problem is not serious, try switching to juice instead of water, or taking in salt with water so that you retain it (instead of passing it through your system). Sometimes I have the same problem, and it seems to come and go. Moderation to your diet might be your best bet, especially if you've been trying to drink enough water lately to make up for the fluid loss. Good luck!
Seems like a strange question, but have you ever had any type of abdominal surgery at all? Whether it be an appendectomy, what have you?