What Causes Stretch Mark Like Lines On Inner Thigh?

I have strange stretch mark-like lines on my inner thigh. They ve been there for about three weeks and I m in my teens. Also have been celibate for 2 years, so I don t think it s sex-related. But they re really scaring me. I m a fairly active female, and am 5 6 140. Haven t gained any weight lately, and the marks are the same texture as my skin. However, they seem to lighten up when I m wearing loose clothes or laying down