I have this same problem. I agree to get blood tested just in case. I have gotten tested for diabetes, etc. because of this, and it all came back normal.

I think, in my case, it is a combination of a bunch of things:
1) sometimes I'm just thirsty because I didn't drink enough water or eat enough water-containing food that day
2) my meds make my mouth dry
3) maybe I ate too much salty food that day
4) when I'm anxious, my mouth / throat feels dry
5) I actually get anxious ABOUT being thirsty. Like, if I don't have a water bottle with me, I get worried that I'm going to to be thirsty, so I get more anxious, which makes me thirsty.

edit: this thread made me thirsty. (seriously though)