According to your history i can suspect your tooth has undergone infection since the cavity was not filled and such a cavity in the tooth exposes the dentin
to oral environment and food lodgement in the area leads to bacterial infection.
The infection in the tooth has gone worse due to its spread to the underlying tissues that has lead to formation of lump like growth or a pus pocket.
The symptoms of pain and swelling needs to be subsided by a course of antibiotic and analgesics before starting treatment.
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,