These look very similar to the lumps Daisy had a while ago, I checked with the vet and she said it was most likely an allergic reaction to the metal in her collar.
It was right where the D-ring sat when the collar was loose on her neck, and the silver metal coating had worn away so there was something in the metal underneath that was irritating her skin.
The lumps cleared up as soon as I stopped the contact with her skin.
I leave her collar off as much as possible now, and replace the collar as soon as the metal coating starts to wear down. I did put a thread on here about it (looking for hypo-allergenic collars), and someone suggested coating the metal with clear nail varnish.