Lump on the back of my hand?!?

Ok so i have a small bump on the back of my hand. When my hand is strait, you can hardly see it. But when my hand is bent forwards, there is this huge bump! It doesnt hurt now, but a few weeks ago it hurt when ever i picked something heavy up, or moved my wrist. When i touch it, It feels like a hard round ball. If you would like any more details, please tell me to put it in additional details! Thanks!