The following is, I believe, the thought process a normal person would have. Let it be noted that I am not a normal person.

1. Normal person (NP) gains weight. Say, 15 pounds or so. NP's pants do not fit.
   
2. NP's boyfriend is bigger than NP. NP's boyfriend's pants were, in the past, absurdly large for NP.
   
3. NP realizes that it would be fairly logical to try boyfriend's pants on and see how they fit now.
   

I've been heavier for months, I only tried Will's pants on today when I was headed to the store and realized that my pants (the only ones that fit) had no pockets.

D'oh!