According to your history i can suspect your infected tooth has gone worse and you have developed a periodontal abscess
due to spread of infection to the underlying tissues.
The lump or swelling you can notice under the mandible is nothing but the abscess cavity and it drains the pus in the oral cavity.
This type of periodontal abscess needs to be drained and compressed in position and then followed by a course of antibiotic and analgesic
will subside the symptoms of pain and swelling.
once the abscess is drained the infected tooth can be treated by root canal
procedure to devitalise it permanently and save the tooth or of the prognosis is poor the tooth can be extracted.
get an x ray done the affected area and approach a dentist
for the treatment.
meantime you can start with antibiotic augmentin
625mg with analgesics two times daily, this helps in subsiding the swelling and pain.
i hope this helps,