on tooth has progressed into periapical area causing abscess
formation.If left untreated it can drain into sinuses
causing extra-oral swelling as in your case.
Usually the swelling subsides,once the pus has drained out completely.
Take complete course of medicines prescribed.
To rule out the chance of the lump being cancerous,biopsy
of the lesion is mandatory.
I would suggest you to get routine dental check up done.
Hope this helps.