Hi. No you don't HAVE to be overweight to have stretch marks.. Sometimes when you are develloping(in puberty) many girls(especially) get stretch marks on their breasts, thighs, but, and hips. Because when you are a kid you have a straight narrow body but when you go through puberty your hips and thighs widen and your breasts get bigger and sometimes it happens to fast and your skin doesn't have enough time to be able to gradually stretch so if you had quik changes in your weight than you might get stretch marks. They will turn skin color faster if you apply pure vitamin E oil everyday before going to bed though. Good luck! You can write to me if you like: cr_rabbit87@hotmail.com