Once again, the real question that should be asked is did this 2 week period of bleeding represent a menstrual period or abnormal bleeding due to some other medical issue that has nothing to do with menstruation.
I'd suspect the latter, especially since you've been havng abdominal pain.
As such, you need to make an appointment with your gynecologist and get this looked into. Another concern should be getting screened for anemia from all of this blood loss. That likely is the reason for your fatigue.
No; the fact that your OBGYN says you have a tiny uterus doesn't not mean you have an abnormally small rectum/anus (and this would play no role in constipation).
For many, two bowel movements a week is perfectly normal. Constipation occurs when having a bowel movement becomes difficult and has nothing to do with how frequently one has one. Neither has it anything to do with how painful an attempt at anal sex was. The assumption that one must have at least one bowel movement a day is inaccurate. How often one goes is dependent on how much fiber and fluid they consume, how physically active one is and can be affected by medications such as pain meds (even over the counter ones)
If you can pass stool easily, you are not constipated. If you find it hard to pass stool, purchase a box of Fiber One cereal, begin by taking 1/2 cup of this daily and slowly work up to a cup. You will notice more frequently trips to the bathroom within a few days of using this product.