What Could Be The Lump On Breast For Months?

Hi, I have had a lump in my breast for approx. 6 months. I have had a ditigal mammogram and ultrasound that revealed nothing but I am still concerned. I am 32 so my breasts are dense and lumpy. I am concerned it may be a lymph node that will not go down. It is right in the upper outside near the armpit. I was just wondering if there was further testing I should get done as I am still concerned. Bothers me to sleep on that side and wearing a bra. My mother had breast cancer 4 years ago. Thanks