2003 Dodge Stratus Coolant Bottle is leaking

On May 23, 2008 I took my car to my mechanic to have a Cam Sensor Circuit replaced and although my car had never ran hot before I took it to him he replaced my thermostat. As of May 31, 2008 my car ran hot, I filled the coolant bottle with antifreeze not realizing there was a leak, the next morning the bottle was empty. I filled it again today and drove 3 miles it ran hot again. I pulled over immediately because I kept watching the needle on the temp gauge, the bottle was empty again. Why is it doing this?