Why don't you just flow with her need and don't take her to any social or school activities for a while. Spring break is here. Just let her be and feel comfortable at home with no demands.

Validate her feelings and listen, until she opens up and tells you something about her fear. Read the book Raising Our Children, Raising Ourselves. With lots of real life examples it explains everything about children's emotions. It explains how fighting it only makes it worse. Flowing with it with love and understanding helps the child open up, understand herself and then you can help.