sound like you are dealing with recurring, foul smelling diarrhea?
I had an experience with alovely little creature that caused foul smelling diarrhea(and vomiting intermitttanly) and it sucked! It is called giardia. Hers might not be anything, but have her checked by your doc, just in case. they'll want a stool sample so be ready for that. check online for more info, and if it is that, you doc will give you an info sheet on it. My child who is now 13 had it for a while when she was five or six, and then I got it, it took my doc almost a month to figure it out. I lost like 20 lbs.
I found the only thing for a raw butt is a warm bath! I hope I did not scare you, please let me know how it goes.
by the way congrats on your next little one!!