Dd1 is 9, slim and very small for her age, a month ago I noticed a few lines on the tops of her legs ( just above the knee) they go across and look like purple lines, they almost look like stretch marks but I'm pretty sure they are not as she is skinny and has not put on weight to cause her skin to stretch, she has gone up taller ( slight growth spurt ). Are they some kind of stretch mark or something else???