agreed on anxiety trigger / stage fright. check this article on stagefright from the great Hal Galper: http://www.halgalper.com/articles/stagefright-and-relaxation/
Hal also has a bunch of stuff about getting "In Flow" while playing that will blow you away. basically the state where you are not thinking, making perfect musical decisions & in the zone.
I used to know a chick who was an agoraphobic, she needed to pee whenever she was away from home. The reason I mention it is because it reminds me of what you're saying - bodily functions are like your excuse to avoid being in high pressure situations.
Was there an incident leading up to this? Do you actually need to crap onstage? Psychosomatic is a word that comes to mind.