Do you feel it tighten/loosen when you squeeze/release it? Sounds like they took you seriously as you had some great tests to check this out. I'm NOT a doctor but to me it sounds like constipation problems. Perhaps you could try Metamucil or Citrucel (both are fibre supplements) for at least 3 months and see if you notice a difference. Be sure to drink lots of water when doing this. I also found this site http://www.bowelcontrol.org.uk/causes.html
which talks all about
the symptoms of sphincter damage. Good luck.