Palpable lump in breast not seen on Mammo or US..

dms5

A little history: I have had a lumpectomy in 2001(left breast) which was benign and a breast reduction in 2004. I found a lump in my left breast in May 2018 and went to go see my Breast Specialist. She said she did not feel it but did a Mammo and US to investigate further. However, neither showed the lump I am feeling. Now today, it seems it has gotten a little bigger. I have a scheduled appt with a different breast specialist on Wednesday. Why would it not show on test such as a US and 3D mammo but I obviously feel it? It baffles me.. any thoughts.