R.J.
It sounds like it could be irritated skin from heat or something her clothes were washed in.
If it doesn't spread, doesn't itch & there aren't any other symptoms (like a high fever) a little Aveeno oatmeal bath would feel nice and just keep the area dry & cool.
If a high fever breaks out (104 or 105) it could be roseola. This usually is benign, it's a red, bumpy rash on the chest & neck & there is a high fever. It lasts about 7 days & then goes away as fast as it came on. It's a virus.