Shoflo, remember to breathe. This will be fine.
Are you able to produce any output at all while you are blocked up? If so, what is the consistency? (Rocks/solid/pudding/liquid)?
A sitz marker study is usually to assess colonic motility, which is tricky without a colon. I guess it can look at small intestinal motility, too, but that isn’t a very likely explanation for a pouch full of stool. Pelvic floor dysfunction could do this, particularly if your sphincter tightens when you need it to relax. Have they done anal manometry, to measure how tight the sphincter is?
Did your pouch work fine before this started? For how long?