Diaper rash

I changed my daughters diaper around 5:30 o'clock everything was fine. All of a sudden around 7 she starts SCREAMING! I've never heard her sound like this before. It was so bad I was actually nervous and almost brought her to the hospital. My husband smelled her said she didn't poop so she kept screaming I decided to check and she indeed pooped. I took her diaper off and it was beat red. I don't know how it got so bad so fast. She has NEVER had a diaper rash so I'm not used to this. What could have cause it so fast her poop was literally burning her. I should mention she has a cold and has for the past two days.