I've noticed that when I walk, I have to walk a little stiff, because if I tried to move freely I can feel my bum cheeks rubbing together. I think this friction is causing stretch marks, as I have noticed red marks going down the inside of the cheeks in streaks (which I recently took to feeling today in the shower, and they felt like stretch marks.) I am a male age 18 and weigh 154 pounds. I was kind of hoping to hear what a doctor might say.