Dr diagnosed external hems 4 weeks ago. had rectal bleeding 3 times last week. rectal exam showed none today. was blood caused by rhoids or cancer ?

Probably: If the blood was bright red and not mixed with stool, it is quite likely that you bled from the hemorrhoids. However to be sure, I recommend that a proctologist or colorectal surgeon start with a simple flexible sigmoidoscopy which visualizes the descending colon and would be good evidence in ruling out internal hemorrhoids, benign polyps, ulcerative colitis,or malignant growths.