Since mammary tumors are often hormone related, nipple discharge is a common symptom of mammary cancer in dogs (and people). Swelling, and inflammation or lumps are also what to look for.
Has China had a mastectomy yet, been spayed? I know that you were planning both, but I haven't kept up on the forum much lately, so I may have missed it. She shouldn't have discharge from a healthy nipple, so I would be concerned and have it checked out if the discharge is coming from a different mammary gland than the one(s) already diagnosed with cancer.
Thinking of you...