I read this post on a board about bounceback weight. Great explanation:
My surgeon kept telling me to expect a 20 pound regain after the honeymoon period. At my 24 month checkup, he told me that it was probably very close. He said that it did not matter that I was exercising and following my food plan. I was ten pounds under his goal weight.
He told me that the body is essentially a very smart animal. That animal had been starved for two years and it had been working on a way to recover from the starvation state. He said that the intestines would become more efficient. The hairlike appendages that hold food into contact with the intestines would increase in size and length. They could hold the food longer so that more calories would be absorbed. I absolutely refused to believe that and told him to stop threatening me with this regain stuff. I said it with a smile and laugh. He said that if I did not experience regain, then there was something wrong with my body's ability to recover from the surgery.
I continued happily following my plan and weighing every morning. I was NOT going to gain anything back and that was that. At 30 months, I started to regain. It was about a half pound a week. Some weeks it was a full pound and one week it was three pounds. I ate less and exercised more. If I had not done that, then I probably would have gained more. I ended up with a 18 pound regain or 8 pounds over goal by the end of year 3.