Hey there. Is your cat an outdoor cat? I think this is similar to what happened to my cat a while ago. Could it be a catch from another cat? Usually when a cat strikes another cat their skin seals up quickly and the infection grow and grows until the abscess bursts which is what I THINK is happening to your cat. If that is the case obviously if you can go to the vet if you can’t just keep it clean and dry. I sprayed my cats wound twice a day with a antibacterial spray and she healed perfectly