Did chemo put you into menopause?

I had just turned 40 when diagnosed, I went into menopause after my second AC.
I have had my bloods checked every 4 months to check that I was in menopause as I was taking Arimidex, bloods showed Estrodial below 3 and FSH high each time ..........fast forward 6 months, 2 3/4 years since going into menopause and my period has returned, so far I've had 2 very normal periods a month apart, now taking tamox, bloods show I'm not in menopause any more.