Is it a young pig? Could she just be burning off energy? I know when my pigs were young (especially really young!) they would run laps of the cage and laps of the hallways during free time... as fast as they could, with sharp turns on the corners, popcorning, etc. They just had so much energy and were having fun! As they got closer to a year they would settle down and not tear around in all directions for no apparent reason.