I have an elongated bump on my neck just below the jaw bone, what might it be?

Depends: Could be submandibular or parotid salivary gland, enlarged lymph node from infection or tumor, congenital cyst, etc. A neck mass must be evaluated so see ent.

Bump on neck: Depends where on the "neck" the mass is located. Just below the jawbone extends from the ear all the way to the chin, so salivary glands and lymph nodes are likely suspects, although tooth related things may also be involved. You may see either an ENT or oral maxillofacial surgeon.