I think your post a little too late for your charity ride.
Sounds more like an irritation. Maybe chemical, fungal or bacterial something or other that gets into overdrive when you provide it the tropical conditions of your sweat soaked body parts.
To keep from getting a chemical irritation, I wash my riding shorts twice. First time with just a very little amount of detergent if any, and a scoop of Oxyclean. Just the original oxyclean as they now make something with perfumes that has a title that makes it sound more powerful but it isn't. Then wash a second time in just plain water, no additions. Hang to dry. Oxyclean is also a fairly good sanitizer so any beasties that might be taking up residence in your chamois will be destroyed or at least reduced in population.
I seldom get any type of chaffing or saddle sore. I think it's because of the way I take care of my shorts/bibs and paying attention to promoting a way for moisture to be wicked away from my skin quickly.
I don't use any chamois butter or creams of any sort. Just my bare butt in the bike shorts and no underwear. I do shave my body hair fairly short. I have lots of it, everywhere but the top of my head. Seems to help keep me cooler in the summer and I dry faster. I think it's because the fabric can get next to my skin and do the wicking thing they designed it to do. Hair seems to get in the way of that and trap moisture.