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.
All good information, thank you for the recommendations! Hopefully your cat stays as healthy as she can.