Hard bone like lump on jaw. It's under my third tooth from back bottom left. .. Pain and swelling but it's been there a long time? Pain now

Evaluation. Advice is to have it evaluated, especially since you now have pain and swelling. All the best...

Lump. Sorry. Can't tell you without an examination. Lots of things cause lumps, some innocuous, some troublesome. Add pain to that mixture and now your chronic problem is becoming an acute problem. You have ignored this long enough. Time to have an expert examine you. Please see your Dentist or an Oral Surgeon.

See a dentist ASAP. Any other treatment like NSAIDs or Tylenol (acetaminophen) OTC, antibiotics is a temporary relief.The tooth abscess you speak of can may spread to your jaw and to other areas of your head and neck. I would strongly encourage you to seek treatment at your dentist ASAP. Take a good care.