They're probably stretch marks. At your age, they're pretty common if you've had any big growth spurts, or you've gained or lost a large amount of weight.
If they hurt, it's because they are - like the name says - due to your skin stretching. Applying cocoa butter, olive oil, or even just a good lotion can help.
I have 'em too, and they showed up right about 13. They're just a natural part of your body, and someday you'll be able to look at them and remember, "Gee, I got those when I was becoming who I am today!" I think they're cool. ;]
"Ooh, I'll settle down with some old story/About a boy who's just like me/Thought there was love in everything and everyone, you're so naive!"-Belle and Sebastian, "Get Me Away From Here I'm Dying"