My first theory:
Your pelvic muscles are tight and clamp down on the your large intestine. The poop sits there and dries out, forming dark pellets that don't move easily to the rectum. When it does get to the rectum it sits there again, refusing to come out. This is because in both the large intestine and the rectum, peristalsis, smooth muscle movement that propels poop forward, can't work optimally. Your poop gets stuck.
My second theory:
People with vulvodynia can have have up-regulated "fight or flight" nervous systems. This shuts down the gastrointestinal track's normal functioning, including peristalsis. A wiki excerpt on the sympathetic nervous system follows at the end of this blog entry.
Note that my first and second theories can co-exist and both are, in my opinion, viable explanations for why someone with vulvodynia is constipated. So what to do? Here it gets gross. Read the next blog.
From Wikipedia on "Fight or Flight"
Sympathetic nervous systemPromotes a "fight or flight" response, corresponds with arousal and energy generation, and inhibits digestion.
- Diverts blood flow away from the gastro-intestinal (GI) tract and skin via vasoconstriction.
- Blood flow to skeletal muscles and the lungs is enhanced (by as much as 1200% in the case of skeletal muscles).
- Dilates bronchioles of the lung, which allows for greater alveolar oxygen exchange.
- Increases heart rate and the contractility of cardiac cells (myocytes), thereby providing a mechanism for the enhanced blood flow to skeletal muscles.
- Dilates pupils and relaxes the ciliary muscle to the lens, allowing more light to enter the eye and far vision.
- Provides vasodilation for the coronary vessels of the heart.
- Constricts all the intestinal sphincters and the urinary sphincter.
- Inhibits peristalsis.
- Stimulates orgasm.