I Have Been Having A Problem With My Bowel Movements

I have been having a problem with my bowel movements for a long time and I was wondering if when I go I don't have a normal looking stool. It is always loose and thin and flat and sometimes on the rare occasion that it is normal there are dark patches in it that I assume is blood. I had a colonoscopy done last year and they found to polyps, one of which was pre cancerous. They want me to have a follow-up in three years. Could I have crohns disease? how worried should I be about this abnormality? Thanks for your time!