So I've now done a MMA lesson twice, the first one was about 2 months ago and I got some pretty bad stretch marks on the left inner side of my bicep, so the part which is against my body when I have my arms hanging to my sides. They are pretty big and now cover my entire inner bicep from the top to bottom and it looks awful. I know stretch marks just fade but never fully and now I've tried a lesson today and I got some on my right arm on the same spot, not as bad as on the left but what am I doing wrong? I did some research and it should not happen normally. Am I doing something wrong or am I missing something? I was decently hydrated. This really makes me think if I should keep going with this sport, I don't want to be a stretch marks walking monster anytime soon. Thanks!