I understand your concern.
A lump in the submandibular area (under your jaw line) can be an enlarged lymph node from a previous infection, either a head and neck infection or a respiratory infection. Most of the time, even after the resolution of infection, the enlarged lymph node persist. Other considerations for a lymph node in the head and neck area are lymphomas and metastatic carcinomas.
I suggest you consult your doctor for proper evaluation.