This was my experience: I also have very dense breasts. I had a 3D mammogram and was told that everything was normal. Two months later I felt what was determined to be a 2 cm nodule in my breast. I was told that I had scar tissue from a previous benign tumor surgery. I did not believe it since my surgery was over 20 years prior. I got a hold of my primary care doctor and relayed my concerns and luckily she approved my getting an ultrasound. I had a rare low grade adeno squamous metaplastic carcinoma tumor that was difficult to identify on mammograms and was only viewed on an ultrasound. Even my local hospital had trouble identifying the tumor and had to send the tumor specimen to Stanford.
Do you think maybe you should insist on an ultrasound?