My daughter, at seemingly random points, would get this can't sit down, raw red, at times even bloody patched rash. And, like, that's not normal. After multiple trips to the pediatrician with zero help, I thiiiiiiink the ONLY search result that was helpful was this mom from Life with Gremlins, and it was a lifesaver.
Now there are a few sites, but the summary is this:
>it's almost definitely dietary-- tomatoes (ahem, pasta sauce?), citrus (juices, or even dried cranberries, too!), even lactose can convert if your kid is already overloaded
>it could be associated with malabsorption; I haven't delved into this yet since it goes away when I remove the food source cause
>the best remedy I found is:
1. Cutting the foods out (pea protein milk was a great alternative!) and doing the BRAT diet (bananas, rice, apples, toast) to help slow the poops until the acidity is under control.
2. Instead of wiping, dab and then throw them into a quick baking soda bath to neutralize the acid; this makes a huge difference. I put like 2-3 inches of water into our bathtub, add 1-3 Tablespoons of baking soda, and let her plan for a few minutes. Then air dry.
3. Finally, apply Neosporin all over the rash, then a diaper paste (like Boudroux's), then on top of those two, put diaper ointment (like Aquafor).
4. Change that baby more frequently than normal (like every 1.5-2 hours or less), letting their bottom air out in between.
This basically stops it in its tracks. Good luck.
