The brake may be binding on that particular wheel - if the brakes are worn right out or you had a cracked disc you would notice much more alarming symptons. Jack the car up and see if both wheels are easy to rotate. another possibility is the CV gaiter has split and is leaking grease onto the brakes. Also the wheel bearing may be on its last legs. In any case get a competent mechanic to look at it right away, there are important safety issues at stake here.