Welcome to HCM.
NOw a lump on elbow reducing to half could gie us many a possibilities such as Infections, Inflammation or Trauma. Arm lumps may actually be boils
or abscesses caused by a local infection, or they can also be caused by other infections like cellulitis
or infection of the tissue and skin. But all of these are painful conditions and according to you your mother's lump is not painful.
So, there can be two possibilities of lump without pain of so much short duration:
1. Hematoma: If the lump bluish in color and is on or near the site of injection then is nothing serious but just the collection of blood due to extravasation of blood at this site
node enlargement: If the lump is skin colored then is due to zoster infection
Both of these possibilities are on the basis of non tenderness of the area. In any of the lump of body we ask the patient or their relatives not to pinch or squeeze the area as it may further complicate the situation.
It is better that you ask the treating doctor gently regarding your query. Seeing your apprehension the doctor will definitely explain you the situation.
Hope this helps,
With best wishes for your health
Dr Sanjay K Kanodia