According to the vet, she probably scratched her ear and nicked a blood vessel, causing a hematoma. They drained her ear, but it seems to be (fortunately slowly) refilling. We're planning to schedule another appointment for Monday, when they said they could basically sew her ear together to prevent it from happening again. I would have guessed they would go for cauterization, but apparently not.
edit; She's also quite underweight (apparently she's six pounds). I thought that was because she's getting old (she's approaching fifteen years) but the vet says she just needs to eat more. We got her some wet food to entice her appetite.
I think when you have an elderly cat, you have to just concentrate on what will give her the best quality of life for however long she has left. In our case the root cause of the ear problem that caused the scratching and the hematoma was never found because we felt that subjecting an elderly cat to an operation would cause her more stress than just living with it. The vet gave her antibiotics and pain killers, and put her on a low dose of steroids that she took for the rest of her life. The hematoma turned into fibrosis and although it looked horrible, the ear stopped bothering her once she got used to it.
With hindsight it was 100% the right decision, and she lived her best life for another 18 months, right up to the last day.