What is wrong with my cat?

I gave my cat a treat and after she had it her mouth she ate it and everything then i went to give her another, but she started chattering her teeth like a person does when they are very cold and she was backing up like she was scared but she was backin up into me, she also started drooling, we got her from the pound, and she is an indoor cat, but sometimes gets out but we quickly get her back in, and this is the first day she has done this, she was perfectly normal before i gave her the treat.