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It is indeed possible for this to be another new breast cancer on that side, since a previous history of breast cancer would really increase your risk of developing another invasive cancer on the normal breast.
Tests to confirm if this lymph node is from a breast cancer is to have it stained for ER or PR (hormone receptors). You may also want to have a mammogram or MRI of the breast since this might identify the origin of this cancerous lymph node.
I doubt if the hives and the allergies have contributory factors to the development of cancer. I hope that your consult with your oncologist will be fruitful and enlightening
Regards and God bless.
Thanks,