Do not mess around with this. It could be anything! Mast cell tumors or squamous cell carcinoma can have this appearance as well as something benign such as a sebaceous adenoma, or as Ireneo says, an abscess. It is best to see your veterinarian. If it is an abscess oral antibiotics are called for. Topical antibiotics can be licked off and are therefore unpredictable in cats. Even if your cat allows bandages to stay on I would not bandage an open draining wound. Covering a wound would allow the infection to incubate thus worsening the condition. It can also cause a nidis for a secondary fungal infection.
Sounds like an abscess. My cat used to get them occasionally from flea bites (she was sensitive). A trip to the vet is called for, perhaps to open and drain the lump, perhaps for antibiotics if it's very bad. When is the appointment?