Don't worry, it's not abnormal to get stretch marks
even if you're not pregnant and you're this young. A large percentage of females will get stretch marks some time in their lives because the size of their bodies changes. ^_^ My sister is skinny as a toothpick (and 19 years old) and even she has stretch marks, so it's not about being fat or skinny what will result in getting them (though if your body size increases, you'll get more because your skin would need to stretch to accommodate the new size).
The bad news? It's permanent. The good news? You can buy products (Bio Oil, shea butter, cocoa butter, etc.) to help REDUCE the visibility of those stretch marks. It'll take a very long time before you'll see any visible results.
Don't be alarm by them. ^_^'