Hello and thank you for your question,
From what I understand, you hit your head on a car door. If that's the case, this injury will NOT cause you a brain tumor or a head cyst.
The most this type of injury can do it to cause a bruise which, in turn, will cause a headache. Your headache is most likely due to the fact that you hit your head (similarly, if you hit your arm, your arm would hurt).
Our biggest worry with any head injury is bleeding inside the brain, however, the type of injury you sustained should not cause that. However, if you develop worsening headache, changes in vision, nausea or vomiting, you need to go to ER. If you don't develop these symptoms, there is no need to go.
I hope you find this information helpful. Please feel free to follow-up with any other questions you might have.
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Best wishes and good luck,