Well, If the lugs were loose torque them to spec, If they were to tight I would check the rest of the wheels. Did the car land on the rotor? If it did you should replace both rear rotors and the lug studs and nuts for reasons of stress and safety. Personally I always replace items in pairs. If I need a taillight bulb, I replace every bulb on both sides at the same time. That's just the way I am.