What does a change in bowel movements mean for my stomach cancer?

I have had stomach cancer for seven years. I have had no treatment except blood tests every few months and iron tablets every day. My bowel movements were black and sticky but lately they have become quite light brown. So can my cancer still be bleeding? My doctor said it is the iron tablets that is causing the black movements, but it does not fit my experiences. Could it be because the bleeding has stopped? I also have more stamina now.