I went out to lunch with my cousin and we were sitting by a window with the sun shining in.
Does sun shine, especially while not wearing sun glasses effect your headaches? I never get headaches, but on some mornings when I wake up with a minor headache, the brightness of the sun will definitely intensify my headache unbearably.
I bent my head down to look at the menu then I noticed twitching or shaking or something in the middle by my nose and forehead. I kept bending my head down and it happened again.
Did this trigger or intensify the headache you were having prior? I don't have any sinus issue, but since you mentioned allergies and sinuses, tilting your head will make for a world of hurt sometimes, the twitching or shaking might be the already-set anxiety of what you believe is a brain tumor, mixed in with the symptoms of sinus/allergy issues. not to mention the shifting of fluid that could potentially be causing some uncomfort.
Honestly I believe you would notice more generalized symptoms had it be a tumor in the brain, such as partial or complete loss of motor functions, numbness on sides of your body and very hard to speak as well, I wouldn't think that a tilt in the head that causes twitching or shaking could be a cause of a brain tumor.
Have you had any xray scans done?