I have a lump just below one of my back teeth.  It is a solid mass and has been there for about a year.  It has recently enlarged and is now sore to the touch.  The lump is not at my gum line but below it.  You can see and feel it easily on the outside of my jaw.  It does not hurt to chew.
Thanks for your query. I have gone through your query. The lump can be a periapical abscess or a cyst, that can occur secondary to tooth infection. This can result in expansion of the bone and produces a marked swelling. Nothing to be panic, consult an oral physician and get a radiograph and confirm the diagnosis. If it is an abscess get the tooth treated with root canal treatment. If it is a cyst then, along with root canal treatment, you have to get the cyst enucleated. Meanwhile, till you consult the oral physician you can take a course of antibiotics like amoxicillin and metronidazole. I hope my answer will help you. Take care