I would say that if your running is too much, meaning distressing your body, it could trigger an autoimmune response...are you bleeding with each bm? But, if you're bleeding, the tissue in the colon is in a state of fragility.
Low in the rectum inflammation won't cause many other symptoms except urgency or constipation.
Do you have hemorrhoids? Did the doc take a look to see if there's anything actually happening in there?