It's possible that it's an allergy. One of my mom's cats developed a scabby bumpy condition down her back legs. It turned out to be an allergy to the crystal litter she had recently switched to. She went back to Swheat Scoop litter and the rash cleared up. I think the vet classified it as eosinophilic granuloma.
Two of my cats have a flea allergy and the bites turn into swollen lumps. They look like a sebaceous cyst, but there's no pus or fluid. It just scabs up and hurts. We take care of the fleas and the bumps and scabs heal.