Ask a Dog Vet and Get Answers to Your Dog Health Questions
Is your dog on flea control, either something like Frontline or Advantage, or something prescription from a veterinarian, like Comfortis, Trifexis, Nexgard, etc.? Your dog may have a flea allergy dermatitis, a bacterial skin infection, or in worst case scenario, she might have some form of mange like demodex. I would suggest having your veterinarian examine her, so they can perform a skin scraping to rule out mange, provide the affected area with some treatment topically, and send you home with the appropriate medications afterwards. If you have an elizabethan collar, you may consider putting this on her, to keep her from causing herself more trauma until she can be seen by your veterinarian.
With respect to her going into her first heat recently, I would strongly suggest you have your dog spayed sooner than later. Female dogs that are never spayed are at increased risk, with every heat cycle, of developing an infection of the uterus (pyometra, a medical emergency), or developing breast cancer (usually malignant). The more times she cycles, the higher the risk of these sequelae conditions developing.