Was it her first egg? If so, are you sure she is a barred rock? barred leghorns and california blacks look a lot like barred rocks, but with white earlobes and they lay white eggs. I thought I had two BRs with my first batch of chicks, but it ended up being one BR and one barred leghorn.
I know, of course, the Barred Holland lays a white egg, but I think, too, they don't have the darker stripe on the front of their leg like a Barred Rock...ummm....not really sure. I know there must be more differences. My mother has one, I would have to check her over.
I don't think she has white earlobes like the Leghorns, though White Leghorns were used to develop them.