D.N.
A friend of mine has this problem (5'9" and was about 115). She is not a heavy exerciser, normal walks and bike rides and a few gym visits a week for strength training classes. She started looking into good fats and learned to love avocado and a couple other items (don't know recall what). She also started breaking up her meals to incorporate a bunch of mini meals in between instead of just a small snack. It took her a few months to change her eating because she would not be hungry for a snack but did need it. She eventually started to gain a few pounds. The last time I talked to her she was able to keep 20 pounds on and her doc is okay with it, though she said he wants her to gain another 10.