See your vet
What it can look like. A general, vet-reviewed example — signs vary and this is not a diagnosis. When in doubt, ask your vet. 
Hot spots in dogs
Clinical name: Acute moist dermatitis (pyotraumatic)
A moist, red, oozing and painful patch that appears fast, with hair loss over it.
Where it shows upcheek · neck · hip · tail-base
Typical patternCheek, neck, hip or tail base.
Linked allergy typesFlea allergy · Environmental (atopy)
Food vs environmentalTail-base hot spots lean flea allergy.
What you can do
Keep it clean and stop your dog licking it; hot spots usually need a vet.
Don't confuse it with
Other causes a vet will want to rule out: bite wound, scald.
This is educational information, not a diagnosis or treatment plan. Always see your veterinarian about your dog's health.
Related signs
Pus-filled spotsKeep an eye on it
Small pimple-like spots with a white/yellow head.
Crusts & scabsKeep an eye on it
Dried serum or blood forming crusts and scabs, often where the dog has scratched.
Scratch woundsKeep an eye on it
Raw, broken skin and scratch marks from the dog's own claws.
Flaking & dandruffTrack it
Dry flakes in the coat and a dull look to the fur.