My mom is currently experiencing constipation, changes in bowel movements and a lot of blood in her stool. What is her possible illness?

She should see . A gastroenterologist immediately... although those symptoms do not mean she has colon cancer (as benign colon polyps as well as hemorrhoids can also cause her symptoms in addition to other GI related diagnoses), she will likely need a colonoscopy for further evaluation and management. Anemia, fatigue and weight loss would make me more worried for her.
These are. general and very serious complaints which MUST be evaluated by a "hands on" session with her PCP who may wish to refer her for endoscopic diagnosis and treatment!!!! The symptoms suggest a mass in her recto-sigmoid area of the colon.....get her evaluated NOW! Hope this is helpful and you can show this to your mom!