my pitbull puppy has something similiar if not the same. some are bigger than others and are mostly grouped on the back of her neck and between her shoulders. They happened overnight and at the beginning it didn't seem to bother her but after 3-4 weeks of having them shes starting to scratch.
the vet did skin scrapings and she tested negative for mange and food allergies. it's not fleas or ticks either. She was put on antihistamines for 10 days and no change. Right now she's getting medicated baths twice a week. Hopefully there will be an improvement.
I'm hoping it's just a heat rash (since it's been unusually hot here in Vancouver this summer). Have you noticed any big weather changes in terms of heat? Did they recently roll around or romp in some plants or bushes?