What Is The Thick Abnormal Looking Lump On My Breast?

I had a breast reduction 8 months ago. There was a lot of bleeding in the left side. I went to have my yearly mammogram a week ago and it showed a lot of fat necrosis. When I went to my gyn yearly visit she found a large hard lump on left side of breast next to nipple. She tried to aspirtate it to see if it would go down. Very little fluid would come out. What did was thick and abnormal looking. She sent it to a pathologist who reported that there were atypical cells. She made me appointment with a surgeon. Why did this not show on the mammogram. I know these kind of cells are not cancerous but could this be the start of cancer. Why a surgeon.