Stretchmarks on back??

Si
So awhile ago I noticed that I now have stretch marks on the right side of my back?? Like I have the once that I usually have on my hips, but on the right side they now go almost all the way up to my armpit. Only on the right side though. I haven't seen any on the left side of my back. Anyone know why I would only get them on one side?? And why on my back??