What Causes Painful Knot At The Knuckle Joint Of Finger?

I have a knot at where my middle finger joins my hand. It has been there for years but my finger was sore when I work up this morning and it was sore when pressed on the pad of my hand just under the knot. Now as the day has progressed my finger down into my hand hurts when I bend my finger. What do you think it may be?