I just completed a sitz mark study last friday. I had the opposite concern, I wanted to make sure I went before starting the test because I didn't want to go any longer than I had to without using a laxative. I took laxatives beforehand to make sure I went, then I took the sitz capsule. Anyway, my doctor said I failed the test miserably. I'm now scheduled for the anal/rectal studies, I forget what he called them. I'm pushing for this operation because I can't go on another year like this. I'm tired of all the laxatives, nausea/vomiting, faintness, sweating, etc. I never know when it will hit, and I'm petrified that it will happen while I'm out somewhere. I wouldn't so much mind messing my pants (embarassing as that would be) but of the pain that brings on the vomiting and faintness, etc.
Anyway, I hope your doctor gets to the bottom (no pun intended) of your problem.