I have a small black spot under my big toenail on my right foot. it's only been there for about 2/3 weeks. i haven't banged it.

This could be a melanoma.: Although this is relatively unlikely, it is the only potentially serious diagnosis that would concern me. I would circle it with a sharpie, but not mark directly over it. Although the nail grows slowly, this may aid your physician in making an accurate diagnosis. A melanoma will not move with the nail. In any case, you should not ignore it.