What Causes Sudden Weight Gain Post Total Colectomy?

I had ulcerative colitis and had to have a total colectomy a year ago. I was very thin for a long time before the surgery and afraid I would never be able to gain the weight again. However, now I m having the opposite problem. I am gaining weight at an alarming rate all of a sudden. I m very bloated and have put on a good 15 lbs in the last 3 months. I m afraid it won t stop. I eat healthier than I ve ever eaten and continue to exercise daily. I ve had my thyroid checked and all results were normal. Why am I gaining so much weight especially in my mid section when I eat better than before and am still exercising? Did the colectomy have anything to do with changing my metabolism? I feel like it almost shutdown my metabolism. I seem to carry a lot of water weight and feel bloated and full most of the time. I don t know what I should do?