Hello and welcome to HCM,
As you said the lump is under your last molar on the gum margin and the molar has broken filling, it can be due to some periondontal infection due to trauma or gingival swelling.
visual examination is necessary to confirm it so i suggest you to visit a dentist
and get the clinical examination done, gingival infection can be treated by deep scaling and root planing followed by a course of medication.
i can suspect eruption of wisdom tooth in your case as the tooth usually erupts in between 18 to 25 years of age but you need to get a x ray
examination done for your jaw that will confirm the exact position of the tooth. i hope this helps you , take care.