Lump just under breast/rib bone

I found a lump 6 days ago under my left breast, just under my rib. I saw my oncologist today and she said she doesn't think it's cancer. I am scheduled to have an ultrasound to see if it needs further attention. She also said it might be a swollen lymph node (apparently there is 500-700 nodes in the body). I had my LAST reconstruction surgery (double mastectomy)2 weeks ago (2.5 yr survivor)and am still concerned. Has anyone else had something similar? I would love to hear your story. Thanks