I had a severe episode like this last week with pain on a level of 9. It came out of the blue, and would strike like a thunderbolt in the bum several times a day, lasting a week or so. Sometimes I thought I might faint. I researched on the Internet and found the following, which I think explains my temporary issue.
There are a number of terms for muscle-based pain in the rectal area, including levator ani syndrome, levator ani spasm syndrome, proctalgia fugax, chronic pelvic pain syndrome, proctodynia, coccygodynia, pelvic floor dysfunction and pelvic floor myalgia. The root cause of rectal spasm is tension in the pelvic muscles including the levator ani muscle. The levator ani is a broad, thin muscle inside the pelvis that tightens and relaxes to aid in a number of bodily functions, including bowel movements. Levator ani syndrome has been described as “a chronic charlie-horse up inside the pelvis”. Pelvic floor muscle spasms can also cause tightness, burning, and a sensation that the rectum is full. Sometimes the pelvic muscle spasm is caused by a trapped nerve or ligament, or some sort of adhesion or restriction in your pelvis. Levator ani pain tends to be constant and centered in (but not restricted to) the levator ani muscle, while proctalgia fugax pain is a short, stabbing pain in the rectum.