2001 Buick Lesabre Thermostat

My car has been using coolant for a while now. I have added some every other day for about six months or so. However, today I drove the car about fiftee to twenty miles down the freeway, (much more than I usualy do) then was sitting in traffic for several minutes. I was surprised to see the temperature needle go beyond the midddle line to halfway beyond that while I was sitting in traffic. When I turned the air conditioning off, it gradually cooled down to the middle line. When I tried to drive it this evening, it started getting hot a lot sooner - just a mile down the road. Is this the thermostat, and if so, where is it located and how do I change it?