It all depends on how she regards food. For example: all throughout my early years, up until my junior year of college, I was around 105lbs. Now my height is 5'6", so I was super skinny, and many people alluded to my having an eating disorder. That was until they saw me eat. I ate a lot (heck, I still can pack it in). It was just that my metabolism was super fast. I tried very hard to gain weight, but it never happened. Now it's slowed down a bit, so I have to be more careful, but I'm still on the thin side.
So my general feeling about teenagers and being skinny is that it may just be their metabolism. But if they always just pick at their food (or usually aren't hungry) or you are suspicious that they are purging themselves after meals, they may have an eating disorder.