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1.The history suggest the possible cause could be
lymph nodal enlargement, next possible cause could be
thyroid mass ( keeping previous thyroid problem in mind)
2.The recent lump under the jaw close to ear possibly enlarged lymph nodes.
3.You don't have to inform the doctor again, as the neck scan has already indicated.
4.The scan will include the needful lumps on the neck.
5.
Ultrasound will detect the nature of lump, its source and extensions. An FNAC can be taken to know the type of cells. Depending on that treatment will be depending on.
6.Lymph nodal enlargement will cause due to infection / inflammation localised or systemic, and presence of any malignancy - all these need to be assessed. Good luck.
Hope i have answered your query.
Any thing to ask do not hesitate. Thank you.