From what I understand, there are level 1, level 2 and level 3 lymph nodes considered in breast cancer. Level 1 nodes are in the armpit (axilla), level 2 about halfway between axilla and neck and then level 3 at the base and up the side of the neck. They are linked together like a string of pearls with clumps of them, like grapes, along those strings of lymph vessels. Perhaps your clearance was all of the nodes at level 1, or 1 and 2, but that this node in question is further up the chain? Eithe way, I hope they get to the bottom of this and that the node is swollen because of a small infection or similar. Did you have any vaccinations, either covid, flu, whooping cough or similar in that arm recently, as that can cause lymph node swelling too?