I went through peri- menopause between 47 to 49 anyway and finally through by 49. The ovary removal will put you into fast menopause but the tamoxifen probably is causing similar symptoms anyway. I had strong family history of ovarian cancer with a BRACA1 gene, which is the one linked to both ovarian and breast cancer (still risk with BRca2 but lesser so). I didn't have the gene though so didn't have the surgery.
I had irregular periods in last year of menopause after the prior year of increased frequency and flow.
Tamoxifen does have some uterine risks so this could be what is causing issues. Is that what they think and doing genetic testing to be sure?