Megan is regressing

While going for a walk yesterday I realized that Megan is regressing. She used to be such a calm, gentle girl. Now she pulls on the leash (NEVER a problem in the past) and is aggressive toward other dogs. It used to be no problem for her to greet other dogs on a walk. Now she growls and snaps. Of course, I need to work with her more. I'm wondering if it has something to do with Shelby being along. Could she be protecting "her" puppy?