Cat tail injury not sure what it is

We noticed this on our male 18 days ago. Vet said keep an eye on it. It shrunk and looked like it was healing (darkening and not actively red). Happened to see it tonight it and it looked larger and more irritated. Trying to get to the vet ASAP but what kind of injury would this be? What kind of treatment is most common for this. He is an indoor cat one of two and haven’t noticed anything off with him. (First two before 2nd two tonight). He doesn’t seem to groom that spot in particular which is why it went unnoticed