Hello dear ,,,,really sorry to hear your problem and your concern ,,,,lets start in systemic manner
First you have
IBS you know it cause
diarrhea secondly you have hemorrhoids which usually bleed so it explain your condition bloody dirrhea
Yes
colonoscopy is wise choice rule of thumb is any person above 35 having change in bowel habbits should have colonoscopy,,,,,, and this condition is not only caused by cancer many other conditions like colonic polyps,peutz jager syndrome etc may also cause it
I think your CBC as you say that doctor has said is good enough,,,so you are safe from cancer ,,,,reguarding D.dimer is raised in many conditions like acute infection ,rheumatoid conditions etc
Take care stay blessed