A small hard lump like that sounds like a resolving abscess. It's possible that tooth has a small crack in it that xray cannot see. If that is the case, the pain won't go away and a root canal is in order to see if the tooth can be saved.
Many times a very small crack below the gumline can hold if it has RTC and a crown. If it gets too wide, the tooth has to be extracted.
You can either have the RCT now or wait and see if the pain and lump completely go away but if there is a small crack, the periodontal ligament pulling on it each time it is used will eventually widen the crack to the point where the tooth cannot be saved. An implant is a good choice if that happens.
Keep your eye on that lump. An abscess can resolve, but many times if the problem (decaying nerve, tooth cracked) is still there, there may be no pain but the bacteria is still silently working on dissolving the bone that holds the tooth in. Unfortunately, small cracks that produce a small abscess at times cannot be seen until RCT is being preformed.
GOOD LUCK with this. If it had been just trauma, the pain would have been gone long before this. Additional treatment will probably be warranted.
My very best to you,