I am similar to you in that I am 5'1 and used to be 115lbs. I currently weigh 100lbs. I look pretty average as I have a small build. I run a half hour each day or at least every other day on the treadmill. Drink green tea and lots of non fat skim milk. It's important to get enough calcium as not enough of it causes you to release calcitriol which stimulates fat storage. Milk also contains leucine which helps build muscle mass.