According to japanese ex naruto fan about chie in meganezaru's tumblr post:
Chie cried when she was practicing it at home. She cried from being heartbroken. Before she could explain it further, she got cut off.
Chie herself said this at the movie premiere and the person who I've asked attended the premiere. So what has meganezaru posted is true.
Hope this helps.
I repeat my question,"why" is she heartbroken ?