Had a root canal done 24 hours ago and just had a pocket of puss burst on gum line right below tooth???

Residual infection: Advise your treating dentist of the situation. You may need antibiotics.

Not uncommon: The fistula is a sign that there is an infection and your body is draining it out through the pimple.

Not unusual: The root canal treatment removed infected tooth material from within the tooth, but did not eliminate infection that had spread to surrounding tissues. Please notify your Dentist. Antibiotic coverage and/or referral to an Endodontists may be in order.