I Am 3 Weeks Post Op From A Vaginal Hysterectomy.

I am 3 weeks post op from a vaginal hysterectomy. Both ovaries in tact. I had very light bleeding the first two days followed by 4 days of terrible constipation which I took milk of magnesia for, under my doctors instruction which lead to 3-4 days of liquid bowel movements. I had a few strong days then fell into cramping in the pelvic area. No bleeding. No fever. On the 2nd day I experienced this cramping, I found it painful to walk. Lower back pain as well. The cramping seems to stay constant throb. What could be the cause of the cramping at this point? Doctor is suggesting a MRI.