
if you think it's the colustrum and frequent pooping, her little body will regulate that again soon. it won't be like that all the way up to the new baby.
in my experience, i've never had colustrum do that to any of my dc's tushes.
2 yr molars would certainly be a suspect and you describe a teething child. that's about the right age too.
has she eaten anything unusual lately? my dh shared his rotel with my dd earlier this week. we noticed her little anus was very red and blistered the next day (poops still solid) which made her dad feel very bad, he was sure it was his spicy rotel. We just kept her bottomless until it healed. But, she wasn't pooping 6x's a day, either. Just once.
As for your breastmilk/colustrum, squirting some of that on your dd's rash is the best remedy

you are doing a great job taking care of your dd. it's so hard to see them suffer, but this is not your fault!