Hi E.,
You sound like me...I too am 5'3 (1/2) small boned (this has a LOT to do with how much you weigh) and couldn't gain an ounce if I tried. In high school I balanced from 97-100 but rarely over 100. UNTIL I hit 21, that was my magic number. When I turned 21 my body changed and I was up to 126. I know to most 126 doesn't sound like much but boy was it a lot when I gained it. I have always ate well and never drank coffee or sodas (I call these waisted callories). My advice to you is not to worry unless you are loosing weight. You are very young and believe it or not you are still growing and changing. Do you work out? The best I've ever looked and felt was when I did a workout program that cut out all my cardio (yes, no cardio). This was hard for me to do since I was rollerblading 3 x week for 10 miles each and running 3-4 times a week for 5 miles a run. When I cut out all the cardio and went trickly to weights I gained muscle. Of course mussle weighs more than fat and I was looking real good at 117-119 and honestly eatting more than ever. If you want to chat you can send me a private email.
Good luck and God Bless
K.
One more thing, have you ever counted your calories? When I went on that workout program I had to count my calories. I found out that even though I was eatting every 2-3 hours and eating very healthy I was only consumming less than 1100 calories per day. You body needs at least 1200 a day to maintain.