Radiologist who did breast biopsy said results were concordant.one year later, no growth/change.new radiologist said it may not be concordant.now what?

It can be confusing: I'm guessing your biopsy was benign, the previous radiologist was satisfied by this, but the one reading your most recent film disagrees. In young women mammograms are very hard to read, and i suggest that if you have a 3d or tomosynthesis mammogram in your area you speak to your doctor about having this done as with this technology it is a much clearer study particularly with dense breasts.