I am tall and I have grown rapidly but now I’m slowing down and they have only started to show up very recently, even though I have stopped growing that much. I haven’t had a rapid increase or decrease in weight and I am pretty thin. I have started to become a lot more serious in my sport as a ballet dancer: these stretch marks have started to show up on my butt and that’s it, the only place. A lot of the sites are only telling me for weight but could this have to do with muscle growth? As a ballerina I have to work on my seat muscles and they are constantly tensed in my class because we have to keep it tight the whole time. Then at home I have to do exercises for the V in my seat and I have only started doing these things a lot recently. Could these be because of how I am exercising it and I guess “wrinkling” it a lot?