Does your daughter stay in a daycare? If so, has something happened there that she possibly hasn't told you about? Are there any new kids in her life - step-sibling, babysitter, cousin, neighbor? Is she playing with any new children in the neighborhood? It's possible any one of these things could have triggered something that has caused her to wet the bed.
Bed wetting in children that have been potty trained for a long time - wo incidence - many times indicates a stress factor and not a medical condition.
I would talk to your daughter - and make sure you listen to what she is saying to see if you pick up on any issues or concerns of hers.
In the mean time, explain that you're switching to pull ups until you're able to help her work through her concerns.