I have a pimple on the bottom of my crown(in my gums)?

If the tooth is already dead cuz I got a crown in the spot and had it root canaled awhile back (and the pimple is below that tooth) what to I do? I%26#039;m not sure why it%26#039;s there if that tooth is dead. And the pimple isn%26#039;t too big just the size of a normal pimple.|||the root canal treatment must have got infected that why the pimple with pus drainage. go to a dentist for x ray %26amp; re RCT of the tooth if possible.