Keep an eye on it
What it can look like. A general, vet-reviewed example — signs vary and this is not a diagnosis. When in doubt, ask your vet. 
Pus-filled spots in dogs
Clinical name: Pustules
Small pimple-like spots with a white/yellow head.
Where it shows uptrunk · belly
Typical patternTrunk and belly.
Linked allergy typesFood allergy · Environmental (atopy)
Food vs environmentalNon-specific (secondary).
What you can do
Pustules usually mean a skin infection on top of the allergy — book a vet check.
Don't confuse it with
Other causes a vet will want to rule out: bacterial pyoderma, mange.
This is educational information, not a diagnosis or treatment plan. Always see your veterinarian about your dog's health.
Related signs
Hot spotsSee your vet
A moist, red, oozing and painful patch that appears fast, with hair loss over it.
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.