Yeah, so here is the TMI section of this discussion. Everyone other [MENTION=345109]corrado33[/MENTION] can stop reading now.

See what happens when you wipe your butt extra clean. Use baby wipes or something. Keep changing the wipe until it comes up clean. Do this while squatting low over the floor, not with your butt over the toilet. This is an uncomfortable topic, but it needs to be said that some people aren't as clean as they think they are.

This might help, and I don't mean to imply that this is your problem, but it might be, and it would be useful to know. I had a different but similar problem years ago, which is similarly embarrassing.