Wow this sounds like me, I just posted about a similar issue. I had one of my front teeth root canaled and crowned, years later I noticed this puss filled bump above the tooth that I could pop as well. My dentist did an x ray and couldn't find anything so I was referred to the oral surgeon, who said it was an infection and I had an apicoectomy done, although it was a 60% chance it may work. I had the apico done a year and a half ago and now I am noticing a hard bump which is larger and seems to be spreading to the gums above the surrounding teeth as well so i'm worried the infection has not just come back but is spreading. I left a message for my dentist today who is out of the office but i'm pretty sure I'm going to have to go back to the oral surgeon to get the tooth removed and an implant done.