What do you think?

my border terrier lola can be a bit of a nightmare when she is having a go at one of my other dogs and i spilt them up i always notice her bottom lip sort of trembling like her bottom jaw slightly going up and down. does anyone know what this is a sign of? is it cos i told her off or is she frightened?
it can be anything that she starts esp with the other bitch like all going outside for a wee