Well… shoot. What a shock that must have been for you. I can only hazard a guess. The radiologist who read my mammogram identified one tiny lesion. While having the lumpectomy, it turned out there were two different kinds of tumors in that spot… both larger than the image would suggest. The sentinel node was clear, however. Does that mean that the other nodes were clear? That's an assumption based on statistical risk only.
The thing about cancer is that it can spread teeny tiny cells into the lymphatic system and the bloodstream, impossible for a radiologist to see or for an oncologist to find in a blood sample. That's why many of us get radiation and/or chemo and/or targeted therapy and/or hormonal therapy… trying to destroy those migrant cells. I'm thinking that, when the surgeon had your sentinel node tested and found lobular cancer there, it was necessary to go back and harvest surrounding nodes.
Your oncologist will recommend a treatment plan individualized just for you to help seek out other migrant cells which may or may not place you at risk for recurrence in the future. Is that a rock solid prevention? No, and that's the hard part of this journey… nothing is. Over the months and years ahead, you may or may not want to delve into those statistics with your oncologist. Just don't despair… some days all of us are brave… other days, not so much… but we're always here for each other (which includes YOU).