What Causes Blood In Stool After Drinking Cranberry Juice?

I am my 85 year old mothers caregiver. She has what looks like bright red blood in her stool. It very sudden, only began today, and I am wondering if it could be from the gallon of cranberry juice she drank today. One of the things that concerns me is that she drinks a lot of cranberry juice every day and we have never noticed this.