Bad sheds are due to too low humidity but if your snake was on antibiotics for the pneumonia then those can force a shed which may also come off in tiny pieces or get stuck. The mite killer "Reptile Relief" is a solution of salts so if you used that to treat for mites then that will cause bad sheds also.
What is the snake's enclosure like? Tank, tub, PVC or melamine enclosure, or ?
Oh and if your snake came with mites then the Petco store is infested, never set foot in it again or you will have another battle on your hands to get rid of them.