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You should not try to drain it. If you see your vet, they will drain the ear and suture it to stop the bleeding. If left alone without vet intervention, the ear will usually stop bleeding on its own and the hematoma will "deflate." It will give the ear a bumpy and unsightly appearance but it's common.
You need to treat any underlying infection that might have caused headshaking.
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You are more than welcome! Glad I could help.
I hope you come back with more questions in the future.