We had this too and it was actually diarrhea. I wasn't sure of it until I went on an elimination diet. When I cut out wheat and eggs she stopped pooping 30+ times a day practically over night. When the constant leaky pooping stopped the rash went away.
You can't tell if it is diarrhea in a diaper because the liquid is soaked right up. I caught some of DDs bad poops in an infant potty and they were definitely watery. The "new" better poops are more creamy.