I Had A Mammogram Today An A Thickening In The

I had a mammogram today an a thickening in the upper part of my breast was found. then my dr. did an ultra sound. she said both showed helthy breast tissue. I had a lumpectomy in oct 2017 an all cancer was removed. but my dr is being cautious and is ordering a mri because she wants to know what this hardening is in the breast that had the cancer. is this normal to do an mri and what could this be that doesn't show up in a mammogram