Strange little lump. Lymph node?

WC3

I had a BMX last year with immediate pre pec expander placement. My skin is thin so a layer of alloderm was placed between the skin and expanders, but this doesn't give me much thickness.

Some time after the swelling subsided, I noticed on the non cancer side, a little lump about the size and shape of a grain of rice, near the edge of the expander near my armpit. The expanders were rigid ones with plastic tabs to hold them in place, so I though this was part of the expander or a wrinkle in it.

About a month ago, I had exchange surgery, and now have pre pec implants for the time being, but the lump remains. I don't think it's the cancer. I can tell it hasn't changed in size, that breast was negative for any signs of cancer on all imaging and the post surgical pathology, and I think seeding from the other breast during surgery would be extremely unlikely as during a BMX, my surgeon starts with the non cancerous breast.

But it is a little lump of flesh and I am wondering if the sentinel node on that side was left intact?