How do you get stretch marks??!?

I have stretch marks on both of my thighs, but like the upper thighs, and not inside. why is this? They're red/purplish marks that are a bit dotty and lines. not a lot though. WHY IS THIS? IM 13 AND HAD THEM SINCE 12, BUT NEVER PAYED MUCH ATTENTION. they are embarring!!! i just realized i cant even wear a bikini! why do i have these?! they look like this: (not on the stomach though) http://www.google.com/imgres?imgurl=http://www.howtogetridofproblem.com/wp-content/uploads/2011/03/stretch-marks-2.jpg&imgrefurl=http://www.howtogetridofproblem.com/how-to-get-rid-of-stretch-marks/&usg=__qrKo5GCRsv2NVT-9hMUX7hNPDMQ=&h=220&w=282&sz=11&hl=en&start=0&sig2=5fmNNtPI3eGep7Wuwq78qg&zoom=1&tbnid=vienEU5SGFZOMM:&tbnh=144&tbnw=185&ei=UdjiTYeGKIbUgQfFtb3SBg&prev=/search%3Fq%3Dstretch%2Bmarks%26um%3D1%26hl%3Den%26sa%3DN%26biw%3D1440%26bih%3D809%26tbm%3Disch&um=1&itbs=1&iact=hc&vpx=299&vpy=135&dur=3793&hovh=176&hovw=225&tx=138&ty=118&page=1&ndsp=29&ved=1t:429,r:1,s:0&biw=1440&bih=809