Don't be fooled by a diagnosis of Eczema. This is often a cop-out answer from a ped doctor that does not want to listen to a Mom's instincts!
My little girl had a HORRIBLE time with a fungus in her diaper. It was bright red and shiny. It did not matter what OTC fungal cream we used (including Lotrimin as recommended by her Ped). As she got a little older, she would scratch it raw. The bleeding on her bum got so bad and she cried so much when we changed her that we took her to the Hospital ER and ultimately ended up needing a script.
She is now 3 and we both recently had a round of something that would not clear with an OTC. We filled the script one last time and both used it. We are both now clear.
It was never diagnosed as ringworm, but it seems that fungus is common in little kids. Sometimes it is an out-growth of the warm moist environment on their skin. And, they can pick it up from anywhere - even the dirt off your shoes if they are crawling on your floor. See a doctor (or get another opinion from a specialist before you go the eczema route) and then try to keep her clean and dry - use lots of powder.
~C.