What happened?

What happened to my sugary sweet girl dripping with love and kindness? Well, she has turned into a sour-faced brat. Now, don't get me wrong, Em is still usually loving and caring, but that girl has got some sass! A perfect example of this played out in front of me today while Em was in gymnastics class. Evidently another little girl and Em were not getting along. Em has been in the same gymnastics class all year, and I didn't recognize the other girl, so she must have recently joined. Em has never had a problem with any of the kids there before. In fact, the opposite has been true; she has been too interested in being friends with the other kids at times that she won't pay attention to her teacher during class. Today, for some reason, this little girl rubbed Em the wrong way, and Em let her know!
From my vantage point, I could tell something was brewing between the two of them. I'd look over and Em would be talking to this girl or they'd be looking harshly at each other. They keep walking over to each others' stations and exchanging words. At one point, Em had her hands on her swaying hips, and shook her head back and forth as she put this girl in her place. But the other girl didn't back down. Finally Em took a step off her balance beam toward the other girl, planted her hands firmly on the girl's shoulders and gave her a big shove. The other little girl stumbled backward landing on her bottom on her balance beam. As Em walked away, she just looked back at her with a sense of justice and continued with her assigned practice activity.
Watching this, I was shocked. I could believe the hip swaying and head bobbing but not the pushing. I'd never seen Em take out her frustration on anyone like that (other than E, of course!). Right there, my sweet baby girl had transformed into a sassy girl full of attitude. I can only image what she will be like when she is a teenager. Help!