It is best to get this checked out at your dentist.
It could be something called Torus mandibularis. Usually no treatment is necessary for this as it is a harmful bony growth but it is best to get it checked by a Dentists asap.
Hope this helps
Dental Nurse
HI,
It is difficult to diagnose the nature of the lump. For this you should see an oral surgeon. Examination regarding the location and size and consistency of the lump and a biopsy will help make the diagnosis.
Hope this helps.