I don't weight that much, I was 115lbs at my heaviest in grade 8 and I got stretch marks, probably in part because of the thyroid disorder I was developing, stretch marks on the boobs to the middle of my back, couple of cm wide and long, purple to pinkish when they first appear, also got them on my knee caps and inner thighs but not as pink, bio-oil or knockoff vitamin E oils work good to diminish the sizes of stretch marks, cocoa butter lotions are said to help too, but I'm not a fan of how sticky they are. They're scars pretty much, if you had a grow spurt or like you said play sports you might get stretch marks even if you're small, weight doesn't matter if your skin isn't healthy.
You don't have to be fat to get stretch marks. I got most of my stretch marks when I was working out and running. Oddly enough the first ones I noticed were on my calves, then the backs of my arms. Don't be embarrassed most people have them and a LOT of women that are into sports do. Try getting a good lotion or oil with vitamin E to rub on them to lessen them if they bother you. Coconut oil also helps. Best wishes