Hello, MN Guest, Thank you for your question. You didn't mention if you had Chemotherapy, but I have a feeling that you did, before the Tamoxifen. Chemo. can bring about menopause in many women. Ask your Doctor to check you for this.
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How does chemotherapy influence the onset of menopause?
* During chemotherapy, women may have irregular menstrual cycles or amenorrhea (disappearance of menstrual periods). Some medications used in chemotherapy may also cause damage to the ovaries, resulting in menopausal symptoms or menopause.
* Menopause may be immediate or delayed, permanent or temporary when triggered by chemotherapy.
* There is no way to accurately determine how or when chemotherapy or other cancer treatments will affect your menstrual cycle.
* Menopause is rarely a sudden response to chemotherapy. When chemotherapy treatments begin, you may notice some menopausal symptoms, but usually the symptoms are delayed for several months after treatment is started. This is natural.
* Menopausal symptoms may last for years after treatment is completed.
http://www.chemocare.com/managing/
menopause_chemotherapy.asp