It sounds like melanosis coli. I had it once on a colonoscopy years ago, don’t know why, never before or since. Read up to a little about it to put your mind at ease, it’s why your Dr. asked if you had been using laxatives. I hadn’t either. If everything else was fine, you’re likely having one of those times our bodies do strange things for no good reason to keep us on our toes. Good luck, Happy Thanksgiving!