Fil
I've been told by naturopaths that your kidneys adjust to the extra water so you will find after time that you don't need to go as much. But water is so essential to your body that going to the bathroom a bit more frequently is a small price to pay.
I think you feel thirsty because your body is telling you you need more water. It's like you've woken it up by drinking more water. It's like sleep: I can get by for a little while on not enough sleep but then I have a good night's sleep and I feel really tired, because my body is saying "mmmm that was great, give me more!". I've often experienced this feeling with water too!
Also, you may want to try kidney cleansing. this will make your kidneys stronger.
Hope this helps!
Fil