Diabetes was my first thought, too. Ds was diagnosed with diabetes last year, and that was definitely a big part of what was going on. (The other thing was peeing a lot, but that's hard to tell when you're also pregnant and drinking a lot
) He was soooo desperate for water! It was like we were being terribly abusive by not giving him water at bedtime.
Ask your dr. or mw for a fasting glucose. It will be probably be very obvious if you have diabetes. With ds, we took it first thing in the am, and it was 303. Pretty obvious.
Of course, a pregnant woman needs a lot more water, so it may just be that. Especially since you're entering the second trimester, when your body starts doubling its blood volume. But I would get it checked out anyway. It's better to know for sure.