I’ve had the same thought from time to time, and I’ve learned to laugh about it.
Look at it this way… what if it is a brain tumor? What if it is the biggest, meanest, most aggressive brain tumor in history, and it’s going to kill you?
Well, you were going to die anyway—you knew that, right? I’m not spoiling the suspense of your life by telling you the ending, am I?—so your plans and hoped-for timetable changed a bit. You’d be going sooner than you expected and maybe (hopefully not) with a certain amount of pain, discomfort and maybe loss of control. You may not get everything accomplished that you had hoped (we never do; I hope you weren’t unaware of this, as well).
If there’s something you can do about that, such as getting the MRI, CAT scan or other test performed, and treatment or surgery to make the thing go away, then by all means do that. But ‘right this instant’ there probably isn’t anything you can do about it except treat the immediate problem: the damn headache (that’s when I have these thoughts). So take a couple of aspirin, lie down in a comfortable and quiet place, and try to get some sleep.
Chances are that you’ll feel better when you wake up, and you can learn to laugh at these fears—and treat headaches effectively in the meantime—as well.