I'm 15, 5'9 And Average Weight. I Haven't Gained Any

I'm 15, 5'9 and average weight. I haven't gained any weight rapidly in the last six months, but for some reason I'm getting tons of new stretch marks, I'm not worried about them. I'm worried about the stretch mark next to my spine, it's about halfway up my back and is purple. I was wondering if it's something I should be watching or should get my doctor to check it out. Thanks!