My three year old won't use the potty and she has a nasty yeast infection!
Motherlove has a great salve for rash and thrush that might help. Plus plenty of probiotics Also a low sugar diet.
We lost my fil a few years ago and our 2yo dd who was close to him took it really hard too. Also we pt pretty early but I expect occasional set backs from them and especially with big changes in their lives. I would work on treating the yeast and helping her deal with life right now and not worry too much on the pt yet.
If it's actually yeast, it's not from the diapers. But, if it's a rash, it could be from the diapers.
You need to treat the yeast infection though. You can do it with regular women's yeast medication that you buy in the store, or ask in your health food store for anything they have that will help. For me, personally, I like the Pearls Yeast kit. But, I don't think it's something you want to give to a child without learning something about it first. http://www.amazon.com/Enzymatic-Therapy-Pearls-YB-Caps/dp/B001V6O39Q