What Causes Onset Of Menopause Post Surgery For Breast Cancer?

Good day. Post breast cancer tumor removal. 7 weeks radiation and started tamoxifen 20 mg daily. No menses post surgery to date. Full swing to menopause. Hot flashes up to 20 per day/ night. Is this normal? My immune system seems weak. Flu twice in last 6 weeks any thing I could do to help.