as per the information provided by you i can assure you that its not cancer at all.
the RC treated tooth developed infection and due to its long standing in your case it lead to a
of infection on your gum which appear like a pimple.
whenever the infection gets accumulated inside the bone around the infected tooth , the pus drains out through this drainage site. this opening is called SINUS
see your dentist
and let him take a fresh X ray
of the tooth in question. depending on the x ray view your dentist can tell you if this tooth can be saved by re -RCT or apisectomy.