My daughter used to get like that after a bubble bath. Rinsing her well down there after bubble baths stopped the problem. Maybe there was some soap residue left behind.
sometimes it may be cause by pin worm.
It could just be irritated from her clothes. Have you changed laundry detergant recently?? She could just be having a little allergic reaction...Is it inflamed? I would go ahead and take her in if it's still red and inflamed. If your daughter is really complaining about it though, I would take her in. Could be yeast infection...it's better to have doc rule out stuff though.