I would take her to the Dr. Not to startle you but I would be more concerned that it was something else. The period would be the least of my concerns. As I said not to startle you but I would be going to the Dr ASAP. I would have also kept the nickers unwashed... but I'm also a bit paranoid.

This was my first thought too. Although I am relieved to hear that you think there is not much chance of something untoward going on I think it's still a good chance to have that talk with her and let her know she can tell you ANYTHING and you will believe her. I would still get her checked out at the GP just in case.
Another thing could be (without getting too graphic) could she be playing around down there herself? I know she's still very young but you never know.

I hope you learn something when you talk to her this afternoon.