I have a bb size knot on my thumb knuckle. it hurts to touch it. it will move with my finger so isnt a knot that doesn't move..what could it be?

commonest lesion is: (1): ganglion, but that does not move that much; (2):nor does a spur from arthritis, which is unlikely as you are too young for that; (3): most likely its a skin tag or some growth in the skin, that is movable over the deeper tissues; (4): other uncommon lesions are, fatty tumor, giant cell tumor, nerve tissue tumor, or from past trauma. Should have it off surgically to confirm its exact cause.