My wife is a naturally (very) small boned thin person, 5'1" at 99 lbs. She needed to get over 100 to get pregnant, so being under 100 probably is too thin for her.

Self image is funny thing when it comes to your weight. I always thought I looked fat in pictures of myself. The first time I saw a picture of myself at my new low weight - I was blown away by how thin I looked. Of course, photos are just the same as everyone else sees you.