Hi, I've never heard of this but I just looked it up online after reading your post, and I don't think you have much cause for worry. Here's the link:
http://www.cdc.gov/ncidod/diseases/parvovirus/B19.htm
Here's why you probably don't need to worry. The article by the CDC says by the time a child has developed the characteristic red mark on her cheek, she probably isn't contagious anymore. So the child you ran into probably was WAY past the time of being contagious, since she was back in school already.
I hope everything is ok. It also sounds like it is more of a worry earlier on in pregnancy, and that they should be able to monitor you okay for now.
Just take good care of yourself this weekend, and try to remain calm! I'm sure everything will be okay.