How to make my bum and legs nicer? (stretch marks and fatty)?

I'm a 15 year old girl and have never been overweight but I have stretch marks on my bum and thighs. Also, I don't know if it's cellulite or not, but they have a really weird tone. I do lots of exercise, hockey, swimming, running, and I just stopped horse riding, so I don't know why my they're like that. I don't wear shorts because of it and it's really annoying, and when I go swimming I get to the pool as quickly as possible. Any help on how to make them nicer and fade the stretch marks would really help. Thanks in advance :)