I have the same problem, I'm also 5'5", and have been hovering around 110 lbs lately (was about 100-105 at your age) ... and whenever I do gain a few pounds it just turns into fat on my tummy
The problem is that I'm 24 now, and I have a hard time finding "grown-up" clothes in sizes small enough for me! I will probably have this problem for several more years at least; my mom was skinny like me until after she had her second child, and my dad still has a 30-inch waist...
One thing I can say is to make sure you exercise: when I started running track in high school, I actually gained weight from gaining muscle mass! After running 3-5 miles 6 days a week, sprinting up hills, and all the other crazy stuff they made us do, I had square calf muscles, very firm thighs, and a flat stomach, it was awesome! Now that I don't exercise and just study all day (I'm a grad student), my legs are skinny again and all the muscle in my legs turned into fat on my tummy, lol
Also, be careful about eating high calorie foods, make sure they're healthy too -- don't just go out and eat Big Macs everyday! Even if you are slender, you can still have a high body fat (if you have no muscle), high cholesterol, and/or other health problems caused by lack of nutrients. But if you're good about eating all your fruits, veggies, whole grains, protiens, and calcium, you can probably splurge on a few high-calorie items as well
(My mom always let me eat all the ice cream I wanted when I was in high school
because I was skinny and I needed the calcium -- I have several risk factors for osteoporosis!) And if you tend to gain weight in your tummy, that is a risk factor for heart problems, so you don't want to start really bad eating habits that you'll just have to break later.
You might try the shakes or bars that NYAngel mentioned; my brother is even skinnier than me (I swear you can count every rib!) and his doctor recommended that he drink Ensure shakes along with his regular diet. He hasn't gained any weight, but he is also struggling with some other health problems right now which is probably preventing him from gaining weight.
Another thing to look into is allergies! When I started college I suddenly lost 10 lbs my first semester even though I was eating fattier foods and exercising less, and at one point I was only 92 lbs! I looked anorexic, you could see every bone in my body and I hated it, I was always wearing clothes that covered everything up, lol. The doctors did a lot of blood tests and such (testing for thyroid disorders, etc.) and couldn't find anything wrong. But when I started taking allergy medication for sinus headaches, I quickly gained all of that weight back -- it turns out that my body was using every calorie it could to fight the allergies! While you may just have a high metabolism, if your lifestyle hasn't changed yet you are still losing weight, you might want to talk to a doctor! (And it may not be allergies, it may be an overactive thyroid or any of those other things the doctors tested me for, I don't even remember.)
Anyways, those are my random thoughts, if anyone knows of a miracle fix that doesn't involve running 5 miles a day I'd love to hear it, lol