hi and welcome,
As you have mentioned your infected tooth has undergone root canal treatment
with a cap coverage. i suspect the root canal procedure is not completed to its last sitting which has lead to spread of infection and formation of lump in the jaw. Bacterial infection leads to abscess
formation which may even spread to spaces of neck if not treated in time.
Antibiotics alone will not help you , i would suggest you to get the abscess drained by a dentist
and root canal procedure completed for the tooth. Unless the tooth is devitalised,disinfected and sealed to apex you may experience pain and infection.
clinical examination and associated radiographs are needed before treatment.
i hope this helps, take care.