the painful fluid filled lump that your are felling around your elbow may be indirectly related to the surgery you had.
the painful lump is most likely "infected olecranon
bursitis" . Olecranon bursa is a normal sheath filled with minimal fluid on the back of elbow . sometimes the bursa fills up with excessive fluid resulting in visible swelling. sometimes it can get infected too.
The reason here might be the abnormal boimechanics of your elbow joint after the surgery. consult your doctor, till then apply cold
compresses and take analgesics SOS.