First, you REALLY need to take her in to see the doctor, so that he can tell you what it is. Until then, since the sores are open, you can use a neosporin ointment with pain reliever built in. Every time she pees, clean her up like you normally would, and use a hair dryer set on cool to help make sure that she's completely dry. Letting her run around naked is actually a good idea, since that limits the amount of contact she'll have with the bacteria and moisture in her diaper, reducing her chances of an infection. Hope this helps, but please get her in to see a doctor ASAP.