My kindergartner is embarrassed by her good grades.?

My daughter is normally thrilled to share her schoolwork and and good grades. However her first report card came home this week -straight As- and she didn't want talk about it. Then when her aunt told her how proud she was G left the room looking hurt and dejected. I don't get it. When asked why, she said she just felt shy. I didn't know she had a shy bone in her body.