What Causes Reactive Axillary Lymph Nodes With Sinus Histiocytosis?

I had a breast reduction done for cosmetic reasons and the pathology report showed no abnormality in the breast tissue but said the axillary nodes taken were "reactive" and had "sinus histiocytosis." My surgeon said these are both benign conditions and was not worried at all, but I am wondering what might have caused the nodes to be "reactive" in the first place and how common a finding this is. Also, does it indicate I need to pursue further testing to figure out what the original cause of the reactivity was? I was due to start my period two days after the surgery, so I'm wondering if the nodes could have been reactive because the period was about to start. I have always been very healthy with no other problems. I am 45.