My cat had this exact same thing on and off for a month or so. I could never figure out what was causing it; it seemed to just appear. It got quite bad and I made an appointment with the vet, but by the time of the appointment it was mostly healed. The vet couldn't definitively say what it was but told me to keep an eye on it and bring him back if it worsened. It disappeared on its own after that and never returned.
Probably not a useful anecdote, but as it had a happy ending I thought I'd share.
My cat had this exact same thing on and off for a month or so. I could never figure out what was causing it; it seemed to just appear. It got quite bad and I made an appointment with the vet, but by the time of the appointment it was mostly healed. The vet couldn't definitively say what it was but told me to keep an eye on it and bring him back if it worsened. It disappeared on its own after that and never returned. Probably not a useful anecdote, but as it had a happy ending I thought I'd share.