I've noticed something odd the past few weeks. I started a new routine, and incorporated some different exercises that I haven't done in a while. Just the other day, I noticed stretch marks on some areas of my shoulders, even though I haven't been on a bulking diet, and my weight has been exactly the same. From what I understand and experienced, stretch marks come from growth of the body, which is why I found this odd. I don't think it is fat, since my diet has been the same, and my cardio has actually become more intense lately. Could it just be the new routine working out different muscles?