It is true that intestinal bleeds while taking an anticoagulant should be investigated by a gastroenterologist. However, it is good to keep in mind that there are many other causes of lower intestinal bleeds that are not caused by cancer.....diverticulitis, ulcerative colitis, internal hemorrhoids, etc., etc.
When such a bleed occurs,, tests such as CT scans or MRIs can help to determine the likely cause. Most gastroenterologists are hesitant to perform a colonoscopy while there is bleeding for fear of possibly causing a rupture of the colon in which case surgery would be necessary. Anytime an intestinal bleed while on an anticoagulant occurs.....whether upper or lower GI...... it is important to be in the care of a gasteroenterologist as well as your physician prescribing the anticoagulant in order to address the problem as needed.....especially if there is a need for a blood transfusion depending on the amount and duration of bleeding.
