Thanks for posting your query at DoctorSpring.com
I am Dr.R.K and I am pleased to assist you.
Some of the causes of blood in stool are hemorrhoids, anal fissures, polyps , inflammatory bowel disease and malignancy.
So if it is blood in stool, it need not be just malignancy only.
Since you are not sure whether it was blood or undigested food particle, I would advise you to get a stool analysis done including stool occult blood test.
Since you have mentioned that there is family history of malignancy, the prudent thing to do is to get a colonoscopy done if not done already.
I hope that answers your question.
Let me know if I can assist you further.