A sore lump can be due to trauma or an infected lymph node. It can be a benign bony tumor. Get an urgent examination. If it does not respond to medical treatment, keeps growing or is very hard, get an X-ray and USG Neck done too. There are many other benign causes like cysts, thyroid gland enlargement, sterno-clavicular joint inflammation and so on which may be the cause.
Remember that if it is relatively painless and just above the left collar bone, it may be Virchow's node and the sign is known as Troisier's Sign which may signify an abdominal tumor and in such a case it has to be confirmed by FNAC or biopsy.