This was my primary symptom when I was first diagnosed with UC--it has now morphed into the opposite when I am flaring. It was explained to be that the colon had become so inflamed that the stool could not pass through and would just remain there and harden. The only thing that relieved me back then was Magnesium Citrate Capsules at night. I would need to take 750--1000 mgs when it got really bad with a really big glass of water. I usually went to the bathroom in the morning. I used to buy 250 mgs and adjust the dosage from there. I couldn't take any probiotics back then. They firmed up my stool and that was the last thing I needed.