Pain that no one can diagnose

So for the past few months, for about a week every month, when I'm standing I get the feeling I have to urinate... very badly. I can't and don't urinate anymore, it's just a feeling. If I sit or lay down, the feeling goes away. I went to a urologist and he really didn't know how to handle it since I have ESRD and don't normally urinate. Since sitting and laying down affected it, it sure seemed like it could (should) be a prostate issue. He checked and said it was fine. He gave me medication for a spastic bladder, but that didn't seem to do anything. Eventually the problem just goes away on its own, but them comes back in a few weeks. The constant reoccurrance is the only thing that makes me think it isn't a bladder infection, and the urologist didn't even lean that way. However, it can't tell me WHAT is going on, and that's very frustrated.

This causes me problems in every day life right now when it happens. Has anyone else experience such a thing?