13m old poo & rash helppppp

*poop pic*. My 13mo has had diarrhea all day, but it was only green. Now there’s a bunch of food in it. Like corn greenbeans and fruit snacks (which I’m assuming she found on the floor LOL bc I do not give them to her only her older sister)

Her butt is BLAZING red. I’ve noticed she is only screaming when poop is on her butt/in her diaper) bc she was perfectly fine running around without a diaper and zinc on. Her butt wasn’t as red when I put her to bed but she pooped in her sleep and it was red when I changed her. What could cause/fix this?!

It’s scaring me.

Thank you in advance!!!