Hi Back, My 84 Yo Mother Has Been Hospitalized After

Hi back, my 84 yo mother has been hospitalized after expeiriencing a MI recently and received stent in her circumflex artery. She is a rather complicated case due to several other conditions such as Afib and other heart issues for a long time. She is in good health and works hard to eat right, and stays very busy with clubs and is not over weight. Thur she was hospitalized due to 6.1 hemoglobin and Drs working to figure out the reason. She received 3 units of blood and remains in the 6's hemoglobin. Tomorrow she may be having colonoscopy. Endoscopy showed no problems. She did just report tarry stool. What else could be causing her to loose blood and be so anemic?