Stretch marks

I don’t understand. I’ve been working out more than ever. I have being eating relatively decent and I’ve never had stretch marks before yet now I have them all of a sudden. It’s really hard for me sometimes because my sister is a model and she is a size zero, perfect body, perfect boyfriend, popular, and beautiful and no matter how hard I work to achieve a healthier body, crap like this happens. Is there any way or anything I can be doing that my help them go away?