15 year old male with stretch marks? HElP!?

I am a 15 year old male and I am fairly fit. I am not ripped but I am muscular. In the past year or so i have lost quite a bit of weight. My body looks fine except for the fact that i've developed stretch marks on my mid-upper back in the last couple months. they came about roughly 3 months ago and were not very noticeable but with time, they have grown darker and bigger. There are a total of 3 and are all roughly 2-3 inches long and maybe 1-3 centimeters thick. If it helps any i started working out at the gym about a month before all this came about in order to get in shape for football. I thought you could only get them from gaining weight or getting pregnant but obviously with my situation that isnt true. Can anyone tell me why i have them, and how to get rid of them? If it helps, i am 143 pounds, 15 years old, and am 5'11"