Not absolutely sure about this one...but are you sure they are not stretch marks? I was about 16 when I first noticed mine. They are common for most people going through puberty because of the rapid growth of your body. They may seem like the end of the world right now, because they are probably red-ish right now, but with time the fade and are barely noticeable. My best advice is to google images of stretch marks and compare. If they are there are certain things (cocoa butter, ect.) that can be applied on to your skin to help with them. And if it is stretch marks, don't be embarrassed...nearly everyone gets them, they are just a part of growing up!