How large is this lump ?
Since when do you have it ?
This is most probably an inflamed submandibular lymph node, that is felt as a hard swelling behind your jaw.
Please try taking OTC Anti Inflammatory medicines like Advil, that might help with this swelling.
I would advise you to see a doctor to have this examined and if needed have a needle biopsy to confirm an infection and to start appropriate treatment.
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