I have had a painful lump on the back of my hand for 2 weeks.  The whole back of my hand turned black & blue.  I did not bump it or do anything to cause it.  I went to urgent care after one week and was told it looked like a contusion and to come back in a week if it is not better.  Its very painful.
The painful lump on the back of the hand could be contusion resulting from bleeding underneath your skin. The commonest cause being trauma. You could have hit your hand on a hard surface and probably forgot the accident, resulting in the contusion. Spontaneous contusion do occur but rarely in patients with bleeding disorders or in patients who are on anticoagulant medications( Chat Doctor.  If you are not on any anticoagulant medications or do not have bleeding disorders, the contusion will resolve by itself. Cool compress and hand elevation in a sling with speed up your recovery. Do consult your physician if the contusion persists beyond three weeks or the intensity of pain increases.