I think you're just forming a cloud in your mouth.

My friend and I do that with plastic water bottles sometimes. :D Just twist and twist until heat from the friction of your hands rubbing builds up inside the bottle, and as it shrinks down, the pressure goes up and as long as there's some stray dust particles or something in there, a cloud-thing comes out once you shoot the cap off. :D It's just the water vapor that condensed.

I think I saw something like this on Bill Nye as well.