At this point it is difficult to say whether it is colon or uterine without the appropriate investigations. A colonoscopy may be required along with a pelvic examination.
The fact there is no blood in the stool is a good sign, and you don't mention pus or mucus so that is also a good sign. At your time of life you are at increased risk of more serious bowel disorders, including cancer but the earlier these are investigated the better the outcome. eg benign polyps in the bowel can become cancerous if left untreated so early intervention is essential. You are a long way from being too late, however I would not ignore these warning signs.
If the symptoms were from the uterus, then again I would not want to leave this unexplored as postmenopausal bleeding can indicate a number of problems and you are just on the edge of falling into this description agewise.
I will not alarm you with all the possibilties, because thankfully most are actually quite rare however I would take this opportunity to review your current state of health with your doctor and put your mind at ease.
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