Argh, just typed out a whole response and the computer ate it!
Wondering if this is a yeast rash? It sure sounds like it. Probiotics are AWESOME but she might need some extra help directly on her skin. I've used powdered probiotics mixed with a small amount of water to make a paste, then smeared it on. I've also used Lotrimin AF cream (or similar product), and a homeopathic "Yeast Guard". The last one was the least successful.
Poor kid, that's got to hurt.
Its hard for us mamas to see our little ones in pain. I hope this is the issue, and it clears up quickly for you!
Take care, and please update!