from  http://hemorrhoid.cn/Info.htm

POST SPHINCTER ABSCESS

Post sphincter abscess, which means literally an abscess behind the sphincter
muscle, is not rare but is a condition which frequently causes the patient
considerable frustration in achieving a satisfactory treatment and cure. The
patient suffering from post-sphincter abscess may do so for years, being told
he has neuralgia, Cocygodynia, or arthritis of the Coccyx (tail bone), and
frequently will use liniments and salves copiously, to no avail.

The characteristic complaint is a backache at the end of his spine (tailbone
ache). After sitting for awhile, he will have some difficulty getting to his
feet. He will often unconsciously sit on one side of his buttocks instead of
sitting evenly on both. There will be pain during bowel movement and a dull
aching sensation afterward.

Because of the deep-seated location of these abscesses, routine examinations
will often fail to identify them. Once the disease is recognized and the
abscess located, treatment brings instantaneous relief as the offending pus is
removed from the cavity.