I hit my hand hard a few weeks ago. i have a sunken knuckle and a hard lump palm side of my index finger. could this be a tendon?

Hard to say.: A sunken knuckle often means a "boxer's fracture." Get checked to see. Only an x-ray can determine of this is more than a tendon injury. If you have a loss of finger function, you may have also damaged your tendon too, and this may need to be repaired to prevent permanent damage. Get seen!

Could be : It could be either a fracture too . Usually with tendon injury finger movements would be affected. Please see see doctor for examination and X Ray .