What Does A Lump Below The Chin Indicate?

I have a grape sized lump that suddenly appeared within an matter of days. It is on the underside of my chin and pretty hard and swollen. Not on a gland. Very painful, although not hot to the touch. No teeth issues, no sensitivity inside the mouth, no fever. Neck glands all seem fine. I’m a healthy, active 44 year old female. I saw a Dr immediately (day 2of finding lump). He seemed quite confused by it and prescribed azithromax 500mg for 7 days. After 2 days of meds still no relief, in fact it seems to be growing slightly - definitely getting harder.