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,
I am sorry, I couldn't answer you right away.
By changes in vision I mean sudden loss of vision or double vision or blurry vision.
Your diet is really not related to your injury or to your symptoms, although if you are hungry, it can cause some headaches and it can make you feel lightheaded.
If you are really worried about how you feel, you should go to ER. It is very hard to give an advice over the computer without having a chance to speak to you or examine you. Chances are you are just fine but if this will continue to worry you, you should get checked out. You are in tune with your body much more than we are, on the other end of the world.