This is a very common problem -- actually TWO common problems. Many spouses become insecure when their partner loses weight, either assuming they are doing it BECAUSE they are having an affair, or that once they are thin they will be dissatisfied with their life/partner and want to "trade up." The other problem many people experience is with overweight partners/relatives/friends. Sometimes it's just jealously or shame -- you've got your act together, they don't. Sometimes it's feeling that the relationship is changing because your habits are no longer similar. Personal relationships get very wrapped up in food, and when you no longer eat the same way, the person left behind can feel ... left behind.
You might want to pick up a book called Fattitudes -- it mentions these problems, as well as having a chapter specifically for spouses/partners to read to help understand what you're experiencing.