It could be a few things. One is an aural hematoma. A hematoma is caused from excessive scratching at the ears (earmites or bad ear infections)The ear flap usually swells up like a pillow. Here's a good page on Aural hematomas - however, the picture is a Dog's ear:
http://www.marvistavet.com/html/body_aural_hematoma.html
The second is an abscess. Cats who are indoor/outdoor cats will develop abcesses from fighting - then the area around the ear will swell up and may even be hot to the touch.
Let me know if you have any questions. I'll do whatever I can to help.
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