Sounds like your carpet needs to be re-stretched. There are tack strips located along the perimeter of the carpet, one uses a 'kick stretcher' to pull the carpet over these tack strips. A pro could do the job in an hour or two easy. Watch out for the tack strips, as they are just a piece of ply or chipboard with many little nails that poke up thru the board, ready to tear your hands, feet, and other body parts.
