Why did you leave her for a few days? That canbe very dangerous, they should only be in the cage for mating, becauseone or the other could badly hurt eachother.
If you give her a nest box too early she could just use it for a litterbox and then it would be unsanitary for the babies. If she is alreadynesting then you can give her the box but I usually give it on day 29.Put some soft grass hay in the cage and make and indentation in it sothat she will be encouraged to make it her nest. She will pull out LOTSof fur from her stomach and that will complete the nest.
Please do LOTS of reading before the babies are born, there are just SOmany things that could go wrong and you need to be prepared.