I am in almost exactly the same boat. Actually, I put on weight after all my treatments were over. Like you, I don't believe Tamoxifen causes weight gain, but it sure makes it difficult if not impossible to lose! I am so angry with myself for putting on 30 pounds - once the chemo was over, I wanted to eat everything in sight. I have to admit that I was very active before the treatments, and have not really worked out in almost 2 years. So I am starting again to use weights, walk, and watch the diet. I have a feeling it will be a long haul.