Stress induced constipation. Shitting bricks occurs in situations where you are so overwhelmed in your thoughts that you can't relax your asshole to take a shit. When you finally push it out, it feels as though you've passed a turd with dimensions similar to those of a brick.
God damn it! That's two days in a row Opie clogged the toilet! Exams must have him spooked. He's literally shitting bricks.