Hi
Welcome to the MedHelp forum!
Depending on its position, a lump in the neck can be a thyroid swelling, a cyst, a lipoma or a gland or swollen lymph node. If it is a swollen lymph node and it is paining, then since you have a history of cancer, this should be investigated. If this doctor is not listening, then consult another. At least a CT scan or a MRI of the lump should be done.
Hope this helps. Take care!
See an Endocrinologist, and also find a new GP.