Hello, welcome to HCM.
When you are asking a question to a doctor, you need to give minimum two parameters one is age and other is sex. Because without these two, one cannot give your answer properly.
The lump you described indicates a supraclavicular lymph
node. You did not mentioned the side ( right or left). Because, a left sided supraclavicular lymph node is always due to malignant cause not otherwise specified.
As, you have a history of breast CA, you must have to be investigated for any lesions.
Do a Fine needle aspiration cytology from the lymph node and it will prove the present condition.