S.E.
Sounds to me like she is getting some hormonal changes happening. There are young girls that get there period before they even get into junior high. Do you see any developmental changes? Have you tried just sitting down just the two of you? Maybe trying a girl day out just the two of you. Just spending time chatting.
Try talking to her best friends mother. If something happened at school her best friend may have come home and shared the event with her mother.
Sometimes as parents we need to just nature take its course and emotions are part of a young girls life.
Hope all works out