It's homework. It's supposed to hurt. It's like a portable prison you can set up anywhere and become magically trapped, bolted to where you sit for hours.
But seriously, I don't know. It's usually something simple that you don't realize until somebody says something. Drinking enough water? Had too much caffeine? I remember sensations like that sometimes when I was in college pounding pots of coffee. Sort of an anoxic, acidic, drowning feeling - like I was at the bottom of the amazon for a few moments. One day somebody was telling me I seemed too caffeinated. They were right!