Aromatase Inhibitors (AIs) cannot be used if you're pre-menopausal but Tamoxifen can be used if you are menopausal.
People swap between AIs as sometimes difficult side effects improve on a different medication. One of them, Exemestane, is a steroidal medication and works a bit differently from Anastrozole and Letrozole. You'll read other names like Aromasin, Arimidex and Femara, which are brand names for these three AIs.
Was your cancer was strongly oestrogen receptor positive?