Hi, dear I have gone through your question. I can understand your concern. You have history of breast cancer. But your block nostril is not due to cancer most likely. It very uncommon thst breast cancer metastasis is nostril. You may have sinusitis. You should go for ct scan. Anti histamines like chlorpheneremine malate or cetrizine is helpful. Consult your doctor and take treatment accordingly. Hope I have answered your question, if you have doubt then I will be happy to answer. Thanks for using health care magic. Wish you a very good health.