may not be due oral pill always. It may be due to some other skin disorder. Since you have been already on tamoxifen
I guess you have been properly treated with surgery and chemo/ radiotherapy or both for your carcinoma breast. A 14 year old melasma may not be due to cancer of breast also. You need a dermatologist
consultation for your melasma management. Tamoxifen must not be stopped.
Dr. Mohammed Ibrahim