Hello. Most likely, Moriah developed a little irritation on her shoulder, began scratching & just made the situation much worse. Sometimes these are allergy driven, other times, a flea , tick or other insect bite can initiate the reaction. Keep the area clean & apply a little over the counter hydrocortisone ointment twice a day. If the area is not quieter looking by Monday, please see your veterinarian. Thanks for using Petco Pet Education Center, formerly Petcoach.