Random stretch marks on lower back?

Alright, im now 17, im 6'4", and ive had these horizontal stretch marks on my lower back since i was about 14. At first i thought they where just from my rapid growth in height. but i havn't grown an inch for about a year and a half, and the stretch marks seem to be getting worse. they cover my entire lower back some being about 3 inches in length. and ive never been overweight or anywhere near it, ive always been the same basic weight. Im only 165lbs for 6'4". i dont know what is causing them, and ive read similar reports of people saying its hereditary , will no one in my family has had stretch marks besides from pregnancies and quick muscle growth, so thats not the answer. Can someone plz help me. im starting to get concerned. =/