Hmmm, we have that problem sometimes. It seems like their poop is more acidy than normal sometimes and we get the bright red, irritated bottom. Are you using any sort of diaper cream on a regular basis? My guess is that your daughter just has very sensitive skin so using some sort of barrier would help prevent the irritation-maybe triple paste or something along those lines? The other thought that I have is that it could be the disposable diapers that you are using. We finally switched to cloth after having problems with irritation from the chemicals in disposable diapers. It definitely depended on the brand so maybe try using a different brand of diapers. HTH a little.