Can endometriosis cause bad rectal pains?

Yes. It all depends on where the endometriosis lies, but if it affects the area behind the uterus known as the cul-de-sac, it can casue rectal pain and can be associated with significant bowel symptoms.
Yes. Endometriosis in the pelvis can cause rectal pain. More likely, the endometriosis can be on the surface of the rectum or it can invade the rectal tissue and can cause severe pain.