Where did this come from?

So out of the blue Miriam has started biting! Last Friday we were out in a public place where she couldn't run loose like a wild animal, so I made her hold my hand.  She got tired of the "dead weight game" not working and bit me! Then yesterday she bit poor Evan on the cheek. He came to me crying and holding his cheek, saying "Sissy bie!! Sissy bie!!" And there was the proof of what he was saying for all to see! Then today in the space of 10 minutes she bit him twice, once on the cheek and once on the back! Where did this come from??