Having a medical/EMT background, of course I think of the 'what if's', but I use my knowledge to my advantage. Of course, a headache could be a stroke, concussion, retna's detaching from the orbits of the eyes, a mazillion things, BUT, since I know what to look for in those things, I can quickly rule them out and call a headache 'just a headache'.
At 11, your daughter may have a headache due to stress, premenstrual symptoms, too much time in front of the t.v. or computer, etc.
If she isn't sluggish, her pupils are equal and reactive to light, has a relatively normal appetite, weighs an appropriate weight, 'acts herself' (minus whatever obvious pain is associated with a headache), isn't leaning her body or tilting her head to one side and she's lucid, responsive, breathing normally, etc... it's JUST A HEADACHE.
People who think the worse (like you, and I don't mean this in a bad way), are educated enough to know it 'could be' something else, but not educated enough to rule out those awful possibilities, if that makes sense.
My point is, don't go to webmd and look this stuff up. If you happen to take med classes, you'll understand where I'm coming from better... I guess preventative measures and knowing what to do if something (ex: a seizure) does happen, you'll feel a lot better being proactive about it.
She's just fine :)