My Sister Previously Had Breast Cancer. She Found A Swollen

My sister previously had breast cancer. She found a swollen lymphic node at the other side of the body at neck and a doctor gave her cefodoxime. She have been using it for thelast 4 days. Today she found two realy small swolen lymph node at the side of the breast that previously cancer, at the interface of the breast and arm. She checking every week and according her they were not there ten days ago. Is these are can be a side effects of cefodoxime? Do you think she should stop taking?