UserX
Sounds like it might be where the sigmoid colon makes a turn and links up with the rectum...it could be something is loosening up in this area or your bowel muscle is just getting stimulated. As more gunk is removed from the colon the muscle is freer to flex and peristalsis becomes stronger.
Then again...you couldn't be pregnant, could you?! The uterus...and bladder too, for that matter...are all located in the general vicinity that you've described.