For The Past Year Or So I've Noticed If I

For the past year or so I've noticed if I let my stool sit in the toilet water for a while, it starts to form a reddish brown cloud around it. It doesn't spread in the water only directly around the stool. No pain, maybe a slight discomfort from time to time with a lot of gas right before I go... I had a stool sample tested and they said it looked fine. Any cause for concern?