How can I explain scars from S&M play to my new girlfriend?

Well I used to be a really depraved person and I was really into S&M for a while. I was friends with benefits with this one girl and I would get her to cut me with a razor while we did it and it has left a few scars. Anyway my girlfriend who I love to death isn't the kind of person who would approve of this sort of thing and neither am I at this point in my life. In fact this is the first time we're having sex and she hasn't seen the scars which are on my chest and back but I don't know how to explain the crazy stuff I used to do. She's very timid and I don't think she would easily understand it. I just need the right way to explain it.