Wake up w/ horrible rectal pain like melon stuck. laxative helps pain but not constipated. only happens asleep. nerve root cyst in sacral spinal cord?

Proctalgia fugax: Your episodes are quite characteristic of proctalgia fugax which is a sudden, severe spasm of the levator muscle in the anorectal area. It is like a charley horse of the rectum and usually occurs during sleep. It lasts minutes and patients often sit on the toilet, without having a bowel movement, for relief. This is a nuisance but not serious. See a proctologist if you have further questions.

Sacral nerve cysts: Sacral nerve root cysts (often called Tarlov's cysts) are RARELY the source of symptoms (but can be). There is NO involvement of the spinal cord, as the spinal cord ends as it comes down from the brain in the upper lumbar spine (unless tethered), and below that level, there are only nerve roots (as a group, called the cauda equina). Your rectal pain is likely not of nerve origin.