Your mom probably is right about it, those white spots are most often caused by an injury to the nail. Also, it might be zinc or other mineral deficiency.
White spots on your fingernails are called leukonychia. They are caused by damage to the internal part of the fingernail. This is usually caused by hitting your nail on something. They can also be caused by an allergic reaction to something you put on your nails or even from frequent manicures.
White spots on your fingernails are from damage to the nail bed. If you chew on the entire nail bed, and not just the tips then it could cause the white spots, but this is unlikely.
It could be any number of things including what your Mother said. It's also something that happens when the nail bed is damaged. Impact that stresses the section where the nail grows will show up later as a spot. Heavy impact (shutting your fingers in a door) tends to show up as dark red spots. They'll grow out with time.
