Is my rectal pain related to endometriosis?

Possible. If you have a history of endometriosis and are having GI symptoms you should definitely see a specialist and also have a colonoscopy. When endometriosis has invaded the small bowel, large bowel, or rectum, it can be very difficult to manage. You want to stay on top of it. That being said, your pain could be for another reason entirely and a GI doctor can help you figure that out.
Can be. Endometriosis implants tend to grow in the most gravitationally dependent part of the pelvis which is the space behind the vagina (cul de sac of douglas) and on top of the rectum. Therefore, rectal or deep vaginal pain during intercourse can be an indicator of endometriosis.