Should I ask for a biopsy of a benign mass?

I (36) had a mammogram and ultrasound done last week because my grandmother was diagnosed with breast cancer at age 39. The nurse called me yesterday with my results. She said I have very dense breast tissue and that I have a benign mass on my right side and told me that I needed to follow up with another ultrasound in six months. But now I'm starting to worry, how did they know for sure that it is benign? Should I be requesting a biopsy? I left a message for the doctor to call me back with these questions, but I haven't heard yet.

Edit: I spoke with my doctor (obgyn) and she assured me she felt everything was ok with my results, but said I can go for a second opinion if I wanted to, and she gave me the name of a surgical oncologist, so that's my next step.