I was told that I need a biopsy in my left breast, but I have no palpable masses?

I am a 57 year old female who has had annual mammograms since 40. On about 5 occasions I have been called back for a diagnostic mammogram and or sonogram. Everything at that point had been normal. This year I was called back to have a sonogram in my right breast after having a 3d mammogram. I have no palpable masses. My Dr. went ahead and ordered a bilateral sonogram. Upon receiving my results I was told that I need a biopsy in my left breast; the one that they had not seen anything on the 3d mammogram and my right breast was fine. I’m confused. If my Dr. Had not ordered the bilateral, would I have been given the all clear? Should I be having sonogram with my annual mammograms every year? Of course I’m having biopsy.
I’m very pro active with my healthcare.