What Leads Weight Gain After Undergoing Total Hysterectomy?

I had a total Hysterectomy cause i had breast cancer 15 years ago and then it came back on the other side after 15years cancer free. i discovered i carried the cancer gene and decided to have a hysterectomy so i didnt have it going into my female parts. I have now gained 20 -25 pounds and cannot drop it. You have any suggestions? I try to eat right and exercise and it wont come off. Please help ?