I Have Been Prescribed Tamoxifen (20 Mg) Daily For Preventive

I have been prescribed Tamoxifen (20 mg) daily for preventive treatment of breast cancer in the remaining breast after having one simple mastectomy a couple of months ago. However, I have started having a brownish, vaginal secretion the last few days. I had a hysterectomy many years ago. Is there a treatment that I should investigate, or is this a normal side effect of Tamoxifen?