Ok, i like to play with my gf's nipples. Cos breasts are just so nice. And she enjoys me doing it to, which is gr8 cos i will do it loads 
Now for the odd bit, when ever i play with her nipples for a while she always gets thirsty! i dont know why, as soon as she gets turned in that way she needs a drink! Its not bad or anything, i just make sure she has a drink when i want 2 play!
does this happen to anyone else?? or is my gf just as odd as me!
i always need water during sexual activities as well.... have noticed that the more nervous i am, the more water i need.... like when i had a steady boyfriend, just put a glass by the bed out of habit, had just a few sips.... but was with a new guy the other day and was very nervous, must have had 3 glasses of water from beginning to end of our activity... then i had to go pee from all the water.... so ok, wasnt very insightful what i had to say, just the way it is for me...
Well I only become thirsty when I'm panting. lol
Your gf is not odd. I don't know about you though.
Seriously I have a very dry mouth. Mainly because I am a smoker and have very bad allergies that I take medicine for. Mix in some nipple play or whatever and I am reaching for my glass of water often.
Like Lily said panting and heavy breathing will make one thirsty. Your doing good by having something on hand for her.