My mom has blood coming from her anus though she has had a colostomy 1 year ago and her bowel isn't connected. what could be causing bleeding?

Part of colon distal to the colostomy site is likely diseased and is resulting in the bleed. She ought to visit her doctor for an evaluation. Colon that is not bathed in fecal matter is often not healthy due to lack of short chain fatty acids in the stool that provide nourishment to the colon/rectum.

If she still has a rectum in place , the bleeding can come from multiple sources including inflammation call proctitis, polyps or tumor in the rectum or inflammatory bowel disease. It would help to know why she has a colostomy. She at least needs a proctoscope by a colorectal surgeon.