I agree, you've probably got a caliper that's seizing on the rotor. There are steel slide pins that the caliper moves on when you apply the brake pedal. If these get corroded, the caliper will stay in contact with the rotor and overheat, eventually warping the rotor. Fix this soon or you'll be changing rotors too. Take the car for a spin, using the brakes normally. Pull over and feel (don't touch) the front wheels. If one is really warm and the other much cooler, that's what's sticking. They will stick on, not off. In an extreme case, you'll see lotsa smoke from a sticking caliper.