Well, IBS and IBD are bowel problems. There is also Crohns Disease and Ulcerative Colitis that are causes for example.
Worst case scenario will always be Colon cancer.
At this point, you are scared of the unknown.
It is now time to put fear aside and tell your story to a doctor.
Hopefully he will recommend having a colonoscopy because that's really the best place to start. If he doesn't, ask for a referral and get it done.
Procrastination has never been known to help an eventual outcome.
Best wishes to you! Paul