Hi Just Had A Colonscopy And Was Told I Have

Hi just had a colonscopy and was told I have Melanosis of the colon caused by chronic laxative use, the only problem is a rarely ever used laxatives. The Dr looked at me like i was lying - which made me a little upset. I've been looking in the internet to see if there is any other reason and the only thing I can up with is Rhubarb and some herbal things. I've never eaten Rhubarb and am allergic to most herbal things so it can't be that. Is there anything that could possibly cause this? I am only on anti-inflammatory meds for arthritis and supplements for that also - Move Free. Please help@