Hi lybrate-user, I can understand you had gone through a lot, but don't worry this infection
is gonna subside. Mostly after root canal treatment
a patient complains of pain
due to two reasons either failed root canal
or food impaction due to defective crown. The way you described the pain, I mean the intensity of pain suggests failed root canal, may be due to missed canal or due to improper sterilisation. This swelling
is due to accumulation of pus, and most probably a gingival abcess due to some calculus
or the cement with which the crown was fixed has gone in between the gums and teeth. Well xray would have given a clear picture, though I would say go for re-rct with a course of antibiotics. No need to drain the pus, swelling will subside after the course of antibiotic. Hope this helps.