it likely won't go down; it's most likely either a sebaceous cyst (oil gland cyst) or a lipoma (fatty tumor.) Neither would go away on their own. It's a very minor procedure to remove it under local anesthesia.
Dear shubunkin, It sounds more like the area was bruised, it could be a lipoma (fatty tumor), or cyst that had some bleeding into it (hematoma) but I couldn't say for sure. As it has been there all of this time, and injury has provoked the change it is less likely to be a serious problem. However if it is not healing and you are concerned ask your doctor about it with your next appointment.