Poor Maggie does indeed have breast cancer, and by the time the time jump following the premiere picked up, she’d gone through a lumpectomy and a first round of chemotherapy.
One of the many cliffhangers in the Chicago Med Season 4 finale concerned Maggie and the likelihood that she had metastatic breast cancer, and it was a cliffhanger that wasn’t 100% resolved in the Season 5 premiere like the end of Connor’s story and April’s possible pregnancy were.