Flat stools, slivers, ribbon stools ect. Could be many different things. It could either be an internal or external himroid. Flat stools can also play hand in hand with IBS although they are not sure what causes this but it is common.
I am no doctor, but from what I have read it is from your IBS. Your white blood cell count would be high if you did indeed have cancer and since all the work came back positive you have nothing to worry about
Hope this helps!