What Does Hardening Of Breast Tissues Indicate?

HI, I've noticed hardening of breast tissue on both of my breasts. Underneath. It doesn't hurt or itch.
I've had two doctors take a look. The both didn't seem to concerned. My "now" doc signed me up for a
mammography (screening). When I mentioned the hardening to the nurse she got very concerned and now they (simon med) are asking my doc to change it to a diagnostic exam. This is scaring me. I've never heard of this before. Should I be scared?