You were put into "surgical menopause" by having your ovaries removed, and are now taking tamoxifen, which can also cause hot flashes.
There are several antidepressants, such as Effexor, that can help. It is very important to select one that will not interefere with the effectiveness of tamoxifen. (See my thread "Tamoxifen Interactions," 10/24/09, for more information about this.) Accupuncture and exercise have also been found to reduce hot flashes.