What Causes The Lymph Nodes To Swell In Arms?

i have had lymphomia twice in the last four years. one in left eye lacrimal gland and next in my left breast. i now have swollen lymph nodes in both right and left crook of my arms. the pet scan showed nothing but i am concerned. i am 64 and have many auto immune disorders including crohens. i am 5'4' and weigh 157.