This happened to us just last week. My youngest is very sensitive, and will get blisters if she is in a poopy diaper for more than a few minutes. She must have gone while in her carseat, and by the time we got home she was bright red, and starting to blister.
One thing that helps us is to grind up some oatmeal ( I use my cofee grinder) to a powder, and then put it in the tub for a good soak.
Hope she is better...