if you work out in a gym or excercise pulling on your legs it is possible to get a stretch mark. all stretch marks are made from skin stretching past it's limit. it looks like a stretch mark. i wouldn't worry about it as it will fade to a skin like color. if you want to quicken this you can use mederma, or some other stretch mark product. if it becomes inflamed or changes in any way then i would see a doctor about it.