Difficulty emptying a J-pouch usually isn’t due to constipation (when the stool becomes hard and difficult to pass). There are various causes and, correspondingly, various solutions. It would help to know how successful the dilation was, what part was dilated, and whether the doctor thinks it will need to be repeated. The pain might be from an anal fissure, especially if the pouch outlet was dilated too aggressively.
For some of the more common pouch emptying problems MiraLax is recommended, but it’s hard to know if this will help you. Good luck!