I gained weight with tamoxifen - about 5 lbs - and I worked out pretty much every day and watched my diet. Also had serious fluid retention and had to take a diuretic every other day.
I lost the 5 lbs within 1 month of completing tamoxifen. Don't take the menopause factor lightly. I was post-menopausal pre BC and was fighting the weight battle then. My doctor told me that every 10 years your metabolism slows down and I either had to work out more or eat less. Unfortunately, she had been proven right every 10 years