In simple terms, the fat cells break, releasing lipid which is broken down into fatty acids which induce inflammation. The inflammation induces further break down and formation of granulomas. These are the bumps that you describe. Some open and drain, some do not. Some cases of panniculitis are associated with systemic symptoms such as decreased appetite, depression, lethargy, and fever.
Treatment is usually prednisone or prednisolone and seems to respond well.
Many go into long term or permanent remission, others may need alternate day therapy for prolonged periods. Some respond to vitamin E supplementation. If localized, and singular, these may respond to surgical removal. I purchased a book about Abyssinians which stated that all plants are poisonous to Abyssinians. The woman I picked the Aby up from said that although most plants are poisonous it doesn't mean the plant will kill the cat but may just make it nauseous.