I use to be a veterinary technician, and it sounds like a "hematoma" which is serum filled "bubble" so to speak. It is caused by the kitty shaking her head, which in turn has ruptured a blood vessel. The reason she is shaking her head now, is because it feels funny to her. You need to take her to a Vet., he will lightly sedate her, lance it so it will drain, and she will be fine.
Take your cat to the vet. He may have had an allergic reaction to the ear mite medication and may need to have his ear drained. Or the ear mites have started to die and have fallen deeper into his ear and the swelling is his bodies way of reacting to that so therefore his ear would need to be flushed by a vet.
Dear Djofkc, I have an outdoor male cat I inherited from a neighbor a few years ago. One ear is deformed from a fight but now he has a swollen ear from undetermined causes. I have no money for a vet and have been washing his ear with hydrogen peroxide and then applying ear mite drops from PetsMart. The swelling of his other ear began yesterday. It felt hot to my touch yesterday but the swelling seems to be subsiding today. When I swabbed his ear the last time brown spots appeared on the cotton plus a hard shell of perhaps an ear mite. I checked on him just now and he's better - he even let me feel his ear.
It is due to some stingy insect. Most probably your cat has got it from some mosquito bite or other pest. And it is an infection. She must be suffering from pain. So you should visit a vet for your cat.