Does the area look like an air pillow on the ear?
What your baby has is called an Aural Hematoma which is caused by broken blood vessels bleeding into the ear flap cause the ear flap to fill with blood. This is caused by trauma to the ear from scratching or head shaking. This will not resolve itself and will need to be treated by a Vet. It will needed to be drained and sutured. If this is not done the ear will eventually shrivel and become cauliflowered. Joan