Brain Slosh

So I wake up this morning (well yesterday now), and it feels like my brain is sloshing around in my skull. And no, I don't drink, so that's not it. But this is a totally unique type of headache that I get once in a while. It does not feel like stress, tension, eye strain, migrane, or other types. Just that my brain is smaller than the space it used to occupy, and therefore it "sloshes" around up there everytime I move. And also it weighs 2-3 as much as it usually does so it has a little momentum behind it.

Why do we get headaches? Why are they local to the head? We don't usually wake up with a pain in other parts of the body. And how does the Aspirin (Tylenol, Advil, Aleve, etc) know where to go to help? Or does it somehow just numb the whole body and the headache get benefits by default.

I keep thinking (as with most other things) that there must be some reason for headaches. Could be just something to slow us down in the morning, so we can comtemplate why we are here, or be grateful for days when we do not have headaches?