What Causes Weight Gain While On Treatment For Cancer?

I am a two year survivorfrom t. of breast cancer. I am 47 year old woman. I am still on a few daily meds one of which is tamoxifen (puts weight on). I am post menopausal & I have osteoporosis. They removed lympnoids which I also take daily lasix for swelling in arm & hand. I have gained over 20 pounds during my cancer treatment & I cant get rid of it. my stomach is so bloated, I feel & look like I am 3 months pregnant . Look forward to hearing from you. Thanks, Sharon from PA