I've only had my ostomy a little over 3 months, so I'm not quite as knowledgeable as some people here, but I've had similar problems with my stoma. I shower every time I take my pouch off to try to keep the skin around my stoma as clean as possible, but I ALWAYS seem to have a rash that's about 1/3 inch wide around it. When my stoma is not active, it's shrink quite a bit, so the opening in my barrier has a lot of gaps. My output is mostly liquid, so it always gets in the gaps and iritate the skin pretty badly.
When I first had my ostomy, I had a tendancy to messure the stoma when it was inactive and end up cutting the whole in the barrier too small which ended up being the cause of the pain that I always seemed to end up with. I was also told that pain that seems to be coming from your stoma can mean that the stoma has detached from the skin or that theres some sort of blockage. In my case, the pain seemed to be a combination of decreased blood from caused decreased motility of my stoma and pressure from stuff backing up.
I hope you get some answers soon, I know how much of a pain it is to not only have these problems, but to not know why.