What Causes Rectal Bleeding In An Elderly Person?

My mother who is 80yrs old had diabetes. She is bleeding from her rectum area. A week ago my sister saw a sore between her vaginal area and the rectum that was white inside. She saw no blood at the site, but there was a lot of blood in the toilet and she is staining her underwear. Today a week later my sister who lives with her seen an awful lot of blood in the toilet but sees no outside sores. My mom is very heavy and I believe she may have overlooked the original sore that she had seen last week. (with diabetes and no treatment I seriously doubt it would heal that fast. She is refusing to go to the doctors, but we believe she should be seen. Any Idea what this could be?