K.P.
If she's "helping out" by changing the sheets, it's not behavioral. For some children, they just aren't biologically "ready" until Kindergarten or first grade. Like the other mothers suggested, invest in some pull-ups and don't get rid of them until she's consistently waking up "dry".
She's got another year or two before sleepovers become an issue, so don't worry too much. You may want to talk with the pediatrician at some point if she's waking up completely soaked and starting to be upset about it!