[mathgeek]

While this is certainly an argument for “how do I lose weight” the relevant part here is that the asker wondered why they started gaining weight, which makes a reduction in daily calorie burn relevant if caloric intake remained roughly consistent. That is a lot of ifs, naturally.

[XorNot]

Could be a reduction in calorie excrement really - i.e. they just started absorbing more of it.

[mathgeek]

Could be any of a number of factors. No way to know.