Why is my car smoking?

I have a 2003 Mazda 6 V6. Yesterday I started it up and it started pouring thick white smoke out of the exhaust. Coolant level was fine and oil was a little low. I added fresh oil and it stopped smoking. When I drove it to work today it started smoking badly again. I checked the oil and coolant when I got to work and both were at the correct levels. I have no 'check engine' light and can't figure out where the smoke is coming from. Anybody have any idea what is goin on?