This is a thread I posted a year ago. I had the same EXACT symptoms you have!
It seems like the smell is coming from your anus, which is why the scent smells like farts, or poop. Your anus isn't tight enough which is letting smells escape. The "butt sweat" is actually moisture from your intestinal lining and your poop that is leaking out. Check to see if the skin around your anus is sensitive or red looking. Please do a lot of kiegels too to fix it. Also cut out meat as much as you can, junk food, and especially dairy which can be the main cause of the odor.
To explain the sweatiness making your odor stronger, it's because when your body gets hot, the smell of your poop and anus area gets amplified when it's heated up. Dog poop on the sidewalk would smell worse on a hot humid day than a cold day, right? And people only smell you sometimes because your poop is near the opening, or your anus is more relaxed in that moment.
I hope you conquer this or at least think of homeschooling while you find a solution. I know this seems like a lot of random stuff and gibberish but I've been through exactly what you are now experiencing and to be three years fecal odor-free, I wanted to share what I know... Best of luck to you.