We recently added a puppy. The puppy and our older dog get along wonderfully except when the puppy invades the older dog's "safe space". When this happens the older dog nips and growls at the puppy. We respond with a firm "NO", which we rarely ever have to do. In fact this really stresses out the older dog to the point where her bottom lip/jaw trembles for about a minute. Has anyone ever heard of this, should we be concerned that this is a symptom of a problem?