I have red/purple lines on the insides of my thighs, they look like scars

I have red/purple lines on the insides of my thighs. They are almost like scars but a weird color and place. I also have smaller ones on my butt. I thought they might be stretch marks, but I have had no major weight gain or loss. I am 5’6 and 125 pounds, no definitely not overweight. I am a little worried because they are just getting darker. I am 15. Anyone know what this is?