What Causes Swollen Lymph Node On Base Of Neck?

I'm a 40yr female with a swollen lymph node at the base of my neck near my spine. I am not sick and have not been. My health is good. History of lymphoma (dad) and breast cancer (maternal aunt). Doctor took blood yesterday and seemed slightly concerned. Could this be cancer?
I am 5'6" and 151lbs my blood presure and pulse and temp were normal.