4 Year Old Has Bright Red Buttocks, Looks Like A

4 year old has bright red buttocks, looks like a hard slap but she says no one slapped her. She was sitting on the edge of a large sandbox to eat lunch outside at preschool. It doesn't seem to be very itchy but she is aware of the redness. It does not look like excema which she tends to get behind her knees. Any idea what would cause this??