Hello. Welcome to Just Answer and thank you for the question.
I am Dr. Sohaib, a Medical Internist (who deals with diseases of all the organ systems of body) and i will do my best to find an answer for you.
There are number of possibilities here.
This lump could be arising from the bone (if it is very hard), from muscles (if this is firm) or from the fats (if its soft to touch)---and this seems to be a most likely possibility.
The tumors arising from the fatty tissue are called Lipomas. These are benign tumors and do not require any kind of treatment at all unless they cause cosmetic disfigurement, or if they result in pain.
Your Primary Care Doctor can diagnose this with a simple clinical examination, and to be absolutely sure about the diagnosis, he can refer you to a surgeon for a needle biopsy from the mass/lump, and subjecting the specimen for a histopathological examination under the microscope will diagnose this with certainty.
At this point, if this is causing pain, you should have this examined by your doctor discussing all the details i have mentioned.
Lipoma can simply be excised/removed and this is a minor procedure.
Wish you good luck.
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I would have been worried too if i were at your place but let me reassure you that this is a treatable condition and carry a good prognosis with appropriate management.