I think it is well known that chemo often puts women into early menopause and the closer you are to natural menopause, the more likely it is. But I've not heard of radiation alone having that effect. Perhaps Deanne is right and it is the shock of diagnosis and treatment that has caused this.
I was 46 when I had chemo and my periods stopped for a while and I had some hot flushes. Then a couple of months later my estrogen levels bounced back and I had a normal period. Then I started taking tamoxifen and I haven't had one since (14 months ago). So I think in my case menopause was triggered by the combination of chemo and tamoxfien.
The other ladies have given some good advice regarding exercise and eating a healthy diet and minimising alcohol consumption. (Must confess I still have my tea and coffe though.) I've not had any major issues - just mild hot flushes, mainly at night. I find getting anxious can also trigger one.
I would just be a bit careful regarding so-called natural remedies for menopause symptoms. I'm no expert but it's possible those sorts of products may minic estrogen and therefore promote cancer. I would check with your oncologist before taking anything. Good luck.