a decent night nappy perhaps? especially is she is a big wetter?

A wool cover wouldnt have caused the NR as it allows the 'area' to breath and the urine to evaporate into the air.
Maybe something foreign placed itself on the nappies while drying outside adn that could ahve effected her. Is she teething? Did she eat something high in acid's that day?

you need something to cause a barrier and keep the 'area' dry from urine and wetness. liner of fleece may help as it will keep moisture away....and as I said before a decent night nappy to absorbe and pull away the moisture before damage is done