As far as I comprehend it, IBS is not initially caused by anxiety, however anxiety has been linked as a trigger for IBS symptoms for some people.
Personally, if something happens that upsets me emotionally (something severe, not just an unkind word or a broken nail!) then the anxiety I feel shortly thereafter seems to quickly send me into a bout of D. If I'm stressed about school, or a deadline, I don't notice a change in my symptoms (sometimes I'm sick, sometimes not... every day is an adventure :P ).
I think anyone who has IBS suffers anxiety every day the second we realize we have to leave the house. :( The "what-if's" are the worst, because you get so focused on the possibility of an episode that you forget to enjoy life. I'm working on it every day to just let life happen, because in the end you're either going to have an episode or you're not... there's so little you can do about it, you might as well stop focusing on it and just deal with what happens.