My mom (age 45) has chronic upper left-side abdominal pain that comes & goes. she also has back pain (upper & lower) & nausea. for a week, she's been constipated & saw a tiny spot of red blood on stool. what are the possibilities of cancer?

Constipation is definitely contributing to your mother's symptoms and efforts should be made to avoid it at all cost, with physical exercise and proper hydration, aside from diet, critical in the process. Other conditions, including cancer, cannot be ruled out and usually the presence of blood in stool triggers the need for an exam (primary care doc) at least with possible extra tests as needed.