This may be because you have been chronically constipated and there is much more to eliminate...or it could be due to the endo. Endo can implant anywhere in the pelvic cavity (actually, they have also located it in lung and brain tissue!) and your symptoms may be the consequence of the endo or the adhesions that develop with it.
I had endo all over my bladder (I saw pictures of it after my lap) and I had been experiencing pelvic pain and burning. Also, if the endo has created adhesions that have formed around your intestines, this might cause some pain after moving your bowels. Adhesions are the consistency of hardened rubber cement when they have been around for awhile, and they can literally 'glue' your organs together. I had a friend with advanced endo who started having pain whenever she sat down. When the finally performed her surgery they discovered that her pain was the result of sitting on an ovary whenever she sat down...that's how bad the adhesions had distorted her internal organs. They had to do a hysterectomy on her because the damage was so extensive and the pain was interfering with her ability to function.