Have worked for vets for almost 10 years and been in rescue for 22 years. All water breeds are more prone to hotspots, and it has nothing to do with how well bred they are.
Could be a fly bite or even a bee sting. You can give dogs Benedryl and it works well for both of those and hotspots. I'm tending to think that's what it is.
Leonbergers, in my avatar, are derived from Newfoundlands. They occassionally get hotspots. We shave them and a good inch around them to allow air to get to the area. I found that the vet wanted to give a cortisone shot. They work but mine always got a rebound hotspot as soon as the cortisone wore off. Benedryl works better to dry them out, and we applied a cotton pad soaked in diluted Hibiclens (found in wound care aisle at the pharmacy). Hibiclens kills all bacteria, viruses, yeast, and fungus. Wet hotspots tend to grow both bacteria, yeast, and fungus. Benedryl dose, per Plumb's Veterinary Drug Handbook, which is what vets use, is around 1mg/pound, 3 times a day. We decrease the amount after the spot has dried out a few days, and then wean them off.