After studying for a long time, why does my brain feel like it "hurts" or is full of fluid?

You overdid it!: After studying for a long time, there's no accepted doctor's note to get you out of class or exam if your brain hurts. Perhaps you had too much caffeine to keep you going? Or too much fast food? Salt? Sugar? Fat? More likely you didn't drink enough water, especially to counter the diuretic effect of caffeine. More likely, you need sleep which you didn't get b/c you were studying.