Ouch! Sounds like rectal spasms indeed...the nerves down there get irritated and you get involuntary muscle contraction that hurts like crazy. If I ever get those I sit in a hot bath for as long as I can stand it. This might sound bizarre but you also might try some yoga. Another option would be to visit a muscular medicine specialist (doctor of osteopathic medicine) and get some manipulative treatments done on your pelvis which would probably help immeasurably!
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