How to beat dehydration?

So I'm 34 weeks and just had a Drs visit and the Dr said I was really dehydrated and I should up my water intake but I don't understand how I could be dehydrated I literally drink water all day long and I don't drink soda or juice very often maybe one here and there like when I go out the first thing I do when I get up is make me some ice water and I refill my cup like 100x a day and I even sleep with a glass of water on my bedside for when I wake up in the night... so my question is if I'm already drinking a ton of water how to i beat this dehydration?